My thoughts on the items mentioned so far, plus a couple other ideas:
Leaf Stick: I think that a really high quality Leaf Joystick might be the best thing to produce. It's has relatively low complexity, with a relatively high demand.
Lightguns: Gun-Con, Act-Labs, Top-Gun are available, but another option would be nice.
Restrictor Plates: I like the physical feel of my Oscar Control's Restrictor Plates, but I think that most of the community seems to prefer the programmability of the 49-way adapters or the new Ultimarc stick.
8-Way Push to Fire Knob: *I* want one, but I'm afraid that the demand would not justify producing them.
Star Wars Yoke: Originals are readily available. Repros have been done, but none with a constant supply.
Positional Guns: I'm not much of a fan of positional gun games, but I bought one intending to make a add-on to one of my games. The problem was it did not have the range of motion for a 27" monitor. I would like to have one designed for a large monitor, as it would enable a lot of games.
Beatmania Table: I've never played the game, but it could be fun ... (When I get my Major Havoc controller, I'm planning to make it's mounting rotatable, so the roller will rotate either left-right or up-down. I'm hoping it will work well with Beatmania while in the up-down position.)
- Mark