Pit would be awesome, lots of projectile attacks and the special weapons he could get from the merchants.
Starman is a must. I cannot stress that enough. Or Plague Rat of Doom.
Little Mac's incarnations could be in there, including his NES and SNES versions.
Somewhat copying Ice Climbers, maybe you could have "Pikmin Army," and pick like 3 colors, and they all co-opt on attacks, but it changes dependent on who you choose. So red, blue, and purple don't function the same as red, blue, and yellow, and so on.
K.K. Slider? YES.
Asianxcore, Yoshi is already in SSBM.
Getting his PM:TTYD incarnation would be cool.
Geno? YES.
I'm sure Wario will be added, along with Waluigi.
Screw Ash, Brock is
where it's at, bitches.
Metaknight would be cool...
I'm still hoping for a Smash Bros vs. series. Where the hell is my Nintendo vs. Capcom already? Goddamn that would be awesome. You could do Nintendo vs. The World and even throw in some Konami and Namco characters. Megaman, Klonoa, Dante, Alucard, Pacman, and a host of others would fit in nicely in that universe. Hell, Sega could bring in Power Stone characters. Or how about Goemon? *pants...tightening*