Since this is in the rifle section I'll start there:
1) I'd like to see someone produce a modular design along the lines of the the Springfield M6 Scout series wherein one could adapt which rifle cartridge barrel mated to which shotgun gauge barrel (have a 12g with a .308 or a 20g with a .223 for example), but make the changes quick and easy, possibly without tools, and at one's discretion, like Contenders.
2) I liked my Ruger PC4/P94 combo so I'd like to see both Glock and S&W make companion carbines for their pistol lines.
3) I'd like to see a good, inexpensive American double rifle, something available in fine woods and blued steel but also in stainless with synthetic.
4) I'd like to see Ruger make a fighting version of their GP100 .357 Mag DA revolver; DA only, GunKote matte black, unfluted cylinder chamfered for moonclips, Tritium night sights, intergral light/green laser, 3" barrel, matching Kydex holster, offered in six-shot .357 and five-shot .44 Magnum.
5) Finally, I'd like a 9mm lever gun, just because.