A new one of these, on a topic I've given a great deal of thought to recently; mechs.
Traditionally, the mech is viewed as a rule-of-cool technological dead end. Everyone loves giant robots, but a box of armor with a turret on it will be far more stable and heavily armored. Boring but practical, the mech just can't compete with ye olde tanks. Or can it?
To preface, we need to look at what role a mech, walker, etc would serve to fill. There are, far as I can tell, two main roles to fill. The first is a direct competitor to the tank, except not. The second is a lightweight armored gun platform intended to support infantry in the manner of an IFV, without the internal space.
Next, we need a few assumptions about how the thing is built. Legs are pretty complicated, but can be broken down to a few main joints. Hydraulics are finicky and easily damaged, but modern or near future high end servos are compact and powerful. Assuming heavily armored joints, the main meat of the legs can be relatively lightly armored so long as it's sturdy enough to continue functioning with a few holes in it. Because anything will need to keep up with tanks/trucks/etc, these legs will need to have some manner of wheel or track mechanism. Finally, legs are still relatively easily damaged, so they need to be easily swapped when damaged.
Looking more at the first style, the spider tank; Under most use, it's just a tank. Legs folded and tracks engaged, it presents a slightly smaller profile than a normal tank, with the option to poke its head up over terrain to more easily fire from a hull-down position, shift to maintain balance during maneuvering, and other such things. Pressed with an obstacle that tracks can't climb, it may be able walk up or down them, and throwing a track can be worked around by shifting weight away from that leg. Ultimately, the spidertank is just tank+, likely optimized for minimal weight for mobility when hitching a ride. If there are other more traditional tanks (such as those that follow the pattern of the Israeli Merkava (MBT+IFV)), it'd share a turret with them.
The second style is the lighter weight mech, weighing a few tons at most. Any smaller and it'd be a battlesuit, and indeed probably is close in size to a heavy battlesuit, though far more broad. The primary aim is to get a weapons platform that can be airlifted or dropped with troops, able to withstand enough weaponry to play bullet sponge while delivering firepower akin to an IFV. Compact size and weight means they can fit places a heavy armored vehicle couldn't. Still, they'd similarly need tracks/wheels/etc to keep pace with trucks and other armor. Could be built in biped or quadruped formfactors; quad is more stable, but necessarily heavier. Whatever form factor it took, most of the time it'd still roll rather than walk if it didn't need the added mobility.
Having legs available makes further possibilities, well, possible. Adding them as landing gear to dropships and similar allows for easier dropoffs of troops in areas that normally would require incredibly skilled, dangerous hovering, or improve the ability to grab and hold payloads steady. Anything with legs can jack itself off the ground for minor repairs.
I've also been trying to figure out how to model these in GURPS, but that's another post.