PLEASE DONT LEAVE US HANGING!!!!
You know, this is just out there, you dont have to use it if you dont want to, but for Power Star 3 you could make Boos come out of Mario, and they form to make Shadow Mario, and Mario and Luigi must both use the power of both stars to defeat the incarnate of evil once and for all. Top it off with a dark setting, add rain with a touch of dialogue, and you got yourself a finale!!!!
BUT this is just me rambling on... at least put it through some thought...
UBER AWESOME FLASH THOUGH!!!!