You guys could probably put a lot in... I use 3 custom builds on one SD (one with all music,characters, and stages replaced) and it only runs me about 800mb/2gb
Personally I would like to see:
Ness have Ninten (earthbound)
Lucas have Claus (even moar)
Marth have Awakening Marth (the Vault model?)
and
Mewtwo to have X&Y (if you ever confirm him
)