Personally, when I think of characters for a Brawl sequel, I like to include both a 'hero' character and a 'nemesis' character from a series.
For example - Mario had Bowser
Link had Ganondorf - (Minda/ Wolf Link should have been a transformation move)
and Kirby had Dedede
when I heard Ike was going to be included, I thought they should have included the black knight - basically like Ganondorf but with higher defense and a sword instead of fists.
Sonic should have had Shadow included as a counterweight character - with all Chaos control powers.
And even if they weren't Nintendo created characters - megaman and zero - both as heroes and then Bass as a nemesis
for Donkey Kong, King. K. Rool should have been included
Samus should have had Ridley - at least in a shrunken form as a nemesis
and just for uping the content - Mario should have been given transformation options from Super Mario Bros. 3 like the raccoon suit for flying or the frog suit
so to summarize:
The Black Knight (Fire Emblem Path of Radiance/Radiant Dawn)
abilities: strong defense and hard to knock back due to armor & powerful sword
super move: teleport to enemies and super slash each one, however only possible if the first enemy is hit successfully
Shadow the Hedgehog
ability: chaos spear - launch a zig-zagging beam of energy in indicated direction / chaos blast - radiate a sphere of energy in all directions from character for a short distance / super speed / spin dash
super move: Chaos Control - drastically slow down time and rapidly attack target with hundreds of punches and kicks
Megaman / Zero (x-series version)
abilities: blaster (megaman) beam sword (zero) - both would practically be clones of Link but megaman would be more like Samus
super powers: super shot - megaman charges up a powerful shot like Mario's fireball and it hits everything in its path / seven slash - zero (samurai like stance) moves in a flash forward towards target and hits seven powerful strikes on enemy
Bass (megaman & bass)
abilities: blaster - like megaman's but with weaker shots albeit fixed with rapid firing capabilities & a powerful punch
super power: pretty much the same as megaman's super shot (although now called Mega Buster) with a different color, but this time, Bass launches 3 shots (one left, one right, and one up)
King K. Rool (donkey kong country series) - maybe
abilities: launch a power punch and can create shock waves from jumping and pressing down - if speed is built up - his body acts like a battering ram, knocking everything in his path out of the way
super power: like Dedede - calls in squads of kremlings to attack enemy characters on fiend
a long post I know but I wanted to get as much in in one post