Preface: GURPS 4e does a lot of things decently well, and Steve Jackson games seems to have kind of stopped caring about the system, so a 5e probably won't happen. Maybe these will develop into homebrew rules, or maybe I'll inspire someone to go "oh hey yeah that'd work" (unlikely).
First and foremost, is rapid fire rules. By my estimation, a trained gunman (Skill 13) at close range (2 yards, no range penalty) against an average size target that isn't dodging (ex, All Out Attacked, or just facing away), not aiming but with an all out attack and MAYBE mechanical assistance (for +1 from each), should have a reasonable chance of getting all rounds on target, at least from a RCL 2 weapon. All totaled, these conditions lead to an effective skill of 13+1+1+RoF bonus (+1 through +4 likely)
With RoF 5-8 (+1 bonus)
Skill 16, RCL2 gun. Against a stationary, non-dodging but threatening (no "quiet range" bonus) SM 0 target. An average roll of 10 will get 4 rounds on target. With RoF 5, a roll of 8 is required for 100% rounds on target. With RoF 6, a roll of 6 gets 100%, and a roll of 4 gets 100% with RoF 7. With RoF 8, rules as written you cannot get 100% on target without additional bonuses.
With RoF 9-12 (+2 bonus)
Skill 17, RCL2 gun. Same target as above, average roll of 10 still gets you 4 rounds, a slightly better roll of 9 gets you 5 rounds on target. On a roll of 3, you can get 8 rounds on target. Without additional bonuses, you cannot achieve 100% rounds on target.
With RoF 13-16 (+3 bonus)
Skill 18, RCL2 gun. Same target as first option, same average roll of 10 now gets you 5 rounds on target. On a roll of 4, you can still achieve 8 rounds on target. 100% continues to be impossible.
Make no mistake, 4-5 rounds on target is no slouch, and against bigger targets getting an extra round or two when you need that damage is easier. But it still seems off that it's literally impossible to get all shots on target.
Suggestions to fix: You get RoF/X shots on target per success. I'm unsure of where the best number for that is, but 5-10 seems accurate. Faster firing guns leave less room between shots, meaning it's easier to hit with more of them if you're on target
Decaying accuracy. When aiming at a specific hit location, if you fail to make the attack you miss, even if you were aiming at a location inside of another (such as the eye or vitals). If you miss by 1, you hit the torso, even if you're aiming for something in another body part, for example the eye (success by 9 over skill before hit location modifier), if just barely missed, should instead hit the face (success by 5) or skull (success by 7), but current rules say if you missed by more than 1, the shot sails past even if the shot was good enough to hit the face. Similarly, if you succeed by enough, ALL targeted attacks hit the same spot (ex, managing an eye shot with 3 successes hits the eye 3 times, rather than eye, skull, face, or somesuch due to recoil)
Suggestion to fix: Tables are bad, every table that must be referenced for regular play slows things down, so a scatter table isn't the answer. That also potentially leads to needing either several tables to avoid shooting someone in the face and having round #2 hit them in the foot. My idea is to have hit location decay by RCL. RCL 2 shot to the eye will hit eye, skull, head if you exactly make the shot for the eye and fired 3 shots. A shot to the hand will go Hand, Arm, Torso (or to the gun, it goes gun, hand, arm).
This would also likely serve to change how the Targeted Attack technique worked, as skill at shooting particular small targets is different from just base guns skill, but I'm not quite sure how to fix it. Easiest way would be to make Targeted Attack a Hard technique, that just buys off part of the hit location penalties for everything. Alternatively, it might need to be priced as a Wildcard technique for balance. Each hit location is still subject to the restriction on TA that you can never go below half penalty.
For example: 2 points (first level) in TA: Guns (possibly a specialization here), means you're at -1 to shoot limbs, a -2 for the vitals, -4 for face, -6 for skull, -8 for eyes, -7 or -9 for chinks in armor. another point means you're still at that same -1 and -2 for limbs and vitals, down to -3 for the face, -5 for the skull, -7 for the eyes, etc. You get very swiftly diminishing returns.
Next up is a minor tweak. Swords are assumed to be Parry 0, as are the vast majority of melee weapons. However, according to Low Tech Companion, everything else can have a guard added to it for a +1 to parry, which swords are exempt from. If swords are default, everything else (axes/etc) should be parry -1. Alternatively, parry 0 is the norm, and most swords with full guards should be +1. Mostly a minor tweak for internal consistency, and it makes swords actually worth their relatively extreme cost. Having trained with Parry 0 Tomahawk and Parry 0 Cutlass, the cutlass is FAR easier to parry with.
Weapon Skills: Having trained extensively with melee weaponry, martial arts, and a number of weapons both true and recreational (air guns, firearms, recreational laser weapons, nerf guns, and water guns, to name a few). GURPS has very little overlap between melee skills, despite all the old training manuals often recommending training in seemingly vastly disparate skills (Ex, learning epee as a precursor to everything from broadsword to quarterstaff). There's also only so many possible cuts using a one handed weapon, no matter the balance. Similarly, there are no defaults between any unarmed combat skills. The finest judo practitioner in the world, of masterful body control, is at no particular superior position to learn brawling than anyone off the street (assuming they aren't incompetent and unable to learn unarmed combat). Similarly, most guns use approximately the same skill. This is handled currently by very gentle defaults between them, but leads to oddities with weapons that do not fall cleanly into a category (Ex, does a glock 18 run on pistol skill when in semi-auto, and SMG in full? What happens if you add a stock, does it still use pistol or is it now rifle?).
Suggestion to fix: My suggestion is to treat them as overall categories with familiarities. One Handed Melee Weapon, Two Handed Melee Weapon, Unarmed, And Guns, or something to that effect. Balanced vs. unbalanced, vastly different lengths, things of that nature would all justify being different familiarities. Now you don't need learn a different skill, or have strange mismatches, when you switch a fire selector, or add a stock, or a grip, or anything else, to guns. Martial arts now default to one another, and even to other melee combat skills, meaning experience makes further learning more readily feasible (sort of clumsily handled using perks to adapt techniques for use with another skill. Skill at unarmed combat translates to both one and two handed swords, for example, albeit with a penalty initially).
Armor/Gear is slightly odd in how it's handled taking damage. Armor in particular. Shields tend to have moderate DR, and enormous pools of HP to soak up incoming damage that penetrates, allowing them to do things like absorb rifle fire but be gradually destroyed in the process. While certain armor is best handled as flat DR with no HP soak, such as thin leather, other armors (ex, kevlar vest) seem to lend themselves to a shield style, providing a bit of DR and a lot of bonus HP to the wearer. DR as written works well enough, and there are optional rules for armor damage elsewhere so this is idle musing on dealing with the problem of a great many hits either doing nothing to armor, or punching through and potentially absolutely ruining the wearer. It adds complexity to track gear HP, which would require a different character/gear sheet that I still need to take a crack at.
I'd like for Basic Set to include stuff for gear of different sizes, partially included in the various Tech books, but expanded on the most in Low Tech Companion 1, at least to a limited extent. It's not really a universal system if you can only find gear for SM 0 yet the options are there to be +/- 1 SM right at the start. I'd also like to see the various Power Ups books updated with stuff from books published later (ex, Gun Fu has a number of realistic perks that are greatly desirable for a trained gunman), and consolidated into a Basic Set book #3, for gritty details.
Similarly, I'd like a book for Making Things. Mostly it would be Low Tech Companion 1 rules, extended forwards to allowing high tech and ultra tech armors and such. 4e used player stats to build robots, but there were no rules for making a robot and pricing it beyond things listed. A Points-to-Money conversion ratio, possibly with modifiers (size, complexity, etc) seems like a pretty decent way to allow you to build anything as stats, and then convert into an approximate market value. While I have some qualms with Spaceships, its rules allow for fairly quick but still in-depth building of a great many things, and I believe could be expanded for general vehicles to some extent.
Speaking of money, money is incredibly powerful. I've heard it repeatedly mentioned that the single most effective superpower is Money and High TL (McThag first raised this problem/loophole/etc to my attention). At any level where you can conceivably buy something with money instead of points, you should ALWAYS do so unless you have a very good reason. Ex, 32 points for the implant 9mm SMG in basic set, vs. 3 (2 for extra arm (weapon mount, -80%) and 1 for Payload so it's not just bolted to the side of your arm) and $500ish bucks, a small fraction of starting wealth at that stage. The idea behind having money completely separate is because of the love for infinite worlds style campaigns, with gunslingers out of time, dimension hoppers blending in, and so on and so forth. This unfortunately doesn't quite work as well as it seems at first glance. For example, past about TL6, nearly everyone without a good reason has access to "Immunity (Polio)", and so on. Accounting for all the vaccines and things adds up to a not insignificant amount of points for free to the average high tech character. At ultra-tech, buying augmentations and implants gets confusing because sometimes they cost money, points, or both depending on when or how you get them.
Proposed fix: This one is actually a fairly simple one for something with so much complexity, and ties into the starting wealth available. My proposal is that there be a conversion factor between points and money, with points specifically allotted to "wealth" use. Probably based on TL, this would for example make a TL9 SMG purchased with points or money equally balanced, or DR from armor vs. DR from points). To make it fair, a given campaign would come with however many points are assumed to be the baseline TL, with characters starting at TL0 and purchasing up to campaign TL (or having bonus points to justify being equally badass with less from lower TLs). Mutants and Masterminds does something similar, with everything costing points based on effect (for example, Batman/Ironman and Superman/Hulk are both top tier characters, with the same amount of points. In GURPS, Batman/Ironman would probably be 500ish with some wealth and high TL, and Supes/Hulk would just be a bajillionty points because he apparently has approximately infinite ST/DR/HP/Laser eyes/etc). Essentially this would remove the Wealth advantage/disadvantage, and instead turn it into a per-point-invested thing. It also helps with inventing, bodymods/surgery, and so forth, having money interchangeable. Robots are incredibly hard to deal with on sale price for ones outside of the listed examples, but if points and money have an easy conversion factor, it's a lot more doable.
Also needed are just a few rebalances; Magic has some powerful applicability at higher TLs in making/breaking things (glitch and repair are both mighty powerful, used inventively), but it's essentially worthless to use magic for anything directly offensive. Meanwhile the reflect missiles spell is utterly ruinously broken at high TLs as written. Just including a couple of basic bits of magitech in the base book could substantially solve this problem. At TL5, a magic gun/gauntlet/whatever boosts effective magery by however much, adding Charge Powerstone and making it an entry level spell rather than one requiring massive Magery to reach to the base book to make it more readily apparent that a bunch of small cheap powerstones can be used like ammunition to fire off spells, and so on.
Innate Attack also seems to be something in desperate need of some balancing, as implanted weaponry purchased via the advantage instead of Extra Arm (weapon mount)+Payload is vastly more expensive. Adding Strength Based (from GURPS Powers, allows you to have an innate attack include the user's strength similar to Claws) helps somewhat, but it's still expensive. This may be fixed with the above wealth modification.
Fear Checks. GURPS fear checks have always been kind of weird, because they're only for the super bad things, except not. Any failure, because of the second 3d6 roll+degree of failure, leads to generally moderately severe results. The fix, other than minor rebalancing of the table (giving the option of acquiring a quirk OR some minor negative effect, as with changing/removing/optioning away the fear-barf), is actually quite simple.
Suggestion to fix: Degrees of fright check. Creep, Spook, Fright, and Fear. Creep checks just use Degree of Failure on the table, for stuff like empty abandoned houses, eerie stuff, and minor things where only the most cowardly of characters are likely to do more than hesitate. Spook checks are minor fright checks, and you roll 1d6 on the table, plus DoF. This is for things deliberately unnerving or frightening, enough that the GM might want to see who hesitates or reacts, a bit more strongly than a creep check. Fright checks are for jump scares, and actual frightening things, at 2d6+DoF. Fear checks and beyond work as normal, and are for the major scares, at 3d6+DoF or more (Terror checks, or whatever they get named, could be 4d6, or so on, to handle mindbendingly horrific things)