I think a new Mystery Dungeon would be pretty amazing.
One thing that I found particularly aggravating in the games, though, is how HUGE a factor randomness is. Like, the same mission could go smoothly, or be a complete disaster, purely based on how lucky or unlucky you are. I dunno, some people might not mind that, but not having that much control over what happens in the game is something I find really stressful. I hate it when crap happens that I can't do anything about. Sigh.
So, if they made randomness a bit less of a powerful factor, I'd be all for a new game.