I think there are things we can take from both games and combine them to make it a longer game.
From the first game, bring back saving toads, scary-scary ghosts (as opposed to Dark Moon's scary-funny ghosts), and multiple boss ghosts, along with rankings based off life preserved when the ghost was captured.
From Dark Moon, we can have multiple mansions and the length of the game. Darklight and flashing ghosts was a creative twist, too.
Either way, if a new LM comes out, the switch will be taunting me more to get it.