It doesn't have to be ground up at all.
Yes, it pretty much does.
If its not ground-up designed for analog movements, all your doing is mapping button-presses to gestures.
MP might allow more reliably sensing of gestures, but it doesn't add to the game play if thats all its for, imho.
The sensor bar, of course, remains unaffected by this tech and is pretty perfect as it standards. (So it really doesn't effect FPS games much at all).
I really think decent motion controls with the standard control was asking for the impossible.
Nintendo games managed to pull it off pretty well.
Metroid Corruption, Wii Baseball.
Zack and Wiki also did it well.
Its simply a case of when developers put the effort in to map what they could sense onto movements,rather then gesture-mapping motions to preset motions. Which hardly ever worked well.
MP gives an extra axis and more sensitivity..which does make some stuff easier. But it isn't the big problem, which has been the same since the wii launched.
The "big" problem that makes good motion controlled games hard is the fact you cant use preset animations with them. Even Nintendo skirts around this issue by not giving their characters arms in the WiiSports games.
Analogue motions of characters arms require games to be built/coded a bit differently, especially when these same motions also have to reflect in enemy's and environment interactions.
Developers will have to learn to adjust and that might take quite a few years. But its not something that can be done as an addition to a game. Its pretty fundimental to the way the engine will work.