I recently went back and played through the New Super Mario Bros. and Galaxy games, and I think Bowser Jr. has a really good chance at making the roster. The plot of both NSMB games essentially revolve around him. Keep in mind that as of Brawl's planning period, Sunshine was the only mainline game he had appeared in.
Here's a Sakurai-esque move set I just came up with for him, and there's none of that implausible Shadow Mario transformation garbage.
General: He's light and very fast. His standard attacks would be things like bites and scratches, not unlike Bowser.
Side Smash Attack: Sideways Paintbrush Swing
Up Smash Attack: Upward Paintbrush Swing
Down Smash Attack: Sweeping Paintbrush Swipes (multiple hits)
Standard Special Move: Fireball [New Super Mario Bros. Wii]
Bowser Jr. rears back and spits out a fireball projectile.
Side Special Move: Green Shell Throw [New Super Mario Bros.]
Bowser Jr. tosses a green shell at the opponent; hold B down to charge this up and increase the speed of the throw.
Up Special Move: Spinning Shell [he retracts into his shell in NSMB]
A faster version of Bowser's Whirling Fortress.
Down Special Move: Paint Puddle [Super Mario Sunshine, Mario Sports Mix]
Bowser Jr. mops the floor with his paintbrush, creating a small puddle of paint that stays down for about 5 seconds. If the opponent comes in contact with the puddle they'll slip. 2 puddles can appear on the screen at a time.
Final Smash: Koopa Clown Car [New Super Mario Bros. Wii]
Control the Clown Car similar to Yoshi/Pikachu/Diddy/Sonic's Final Smashes. Press A to spin and B to spit a fireball.