I say add these:

Wario, Waluigi, Diddy Kong, Sonic, and Megaman for sure.

I never liked SSBM too much (I think it's overrated) but if they improve on the game while adding new characters, I'll get it.

I want to see Ganondorf be able to charge those energy balls and throw them at an enemy.

Make the system more advanced, like give Link the ability to change swords or shields, like Soul Calibur... sort of.

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Technoviking does not dance to the music; the music dances to Technoviking.