Regarding compatibility with instruments, I'd love to see Harmonix release something compatible with the Ion Drum Rocker. That thing is huge but Ronalopolis won't get rid of it because it's collectible and those other drum sets are a bit flimsy for someone who has actually played drums in the past. If we could just get another brain for it, that would be awesome. (They sold it with the ability to get brains for other consoles back when it was originally released, so I think it would be possible, if not cost effective.)
I'd also love to see a way to play my Beatles guitars on a new system, but I think that's much less possible, since there were 360 and PS3 versions originally.