I've got several gun designs I want to play around with when I have my workshop set up. The simplest is a slam tube/guerrilla shotgun, improved with an ejection mechanism and a shell side-saddle. Tube-in-tube simplicity, that can be fired as fast as you can single-load a pump shotgun. The next is an intermediate carbine that brings Uzi level simplicity to 5.56. It'll trade accuracy for simplicity and ease of manufacture, but it'll also make things like the AK look needlessly complicated for doing the same thing.
But the first, real project I have, full complexity and capability? Going to pretty much be a pistol(?) carbine version of the Moon Marines SMG. .30 carbine, mp5 style collapsible stock, feeds off M1 Carbine mags out of the grip. Having an S2k, I can attest to just how handy a carbine with the magazine in the stock is, but I want one that hits with some oomph, because nobody makes carbines in 10mm auto or other such magical calibers that I"d never be able to find to feed to the gun anyway.
Probably be a while before I've moved on to somewhere that I have a proper garage to turn into a workshop, but you'll be seeing progress when things get started.