I have been thinking that perhaps a 9mm bullpup carbine would be marketable. If you build it barely within the minimum 26" OAL and 16" barrel it would be extremely compact. Kinda like the
Tavor
-Semi-automatic
-offer in 9mm, 40 S&W, .357 SIG, 10mm, and .45 ACP if possible
-perhaps UZI or Thompson magazines could be utilized
(or popular pistol mags like Kel-Tec does)
-gas piston operated not blowback
-steel upper reciever, ploymer lower
-good trigger (I was thinking perhaps a chain that would PULL the hammer release rather than PUSH)
-Picatinny rail + good iron sights (perhaps AR posts?)
-adaptable for most of the goodies that people like (laser sights, foward grips, lights ect...)
-reasonably light 5-7 lbs. range would be nice
-sleek futuristic appearance (thinking of the simplicity of the Tavor or P90)
-Price would need to be $500-700 range I think