I know many people have had problems with the Augs gas bolt I think, something about it falling off easily. Someone with experience with augs could probably explain better.
replacement RDS for P90 users. doubt you'd be able to make some at a reasonable price but hey worth a shot.
replacement nozzles for TM shotguns, i hear they can break easily and are hard to find replacements for.
replacement parts for KSC USPs, hear the slide stop wears down after a while, or a part of it does.
hell just look in the doctors section, and see what parts people are constantly complaining about or losing/breaking. should give you some ideas on what people want/need.