I never liked Bowser Jr. himself, but he's definitely deserving. He was in the Galaxy games. He was in the last four NSMB titles. That's a pretty impressive résumé.
And when he fights in this:
You have a lot of interesting potential. He could drill into opponents with the propeller bounce. He could lob Bob-ombs and Mecha-Koopas, fire Bullet Bills and Torpedo Teds, and drop Big Steelies. He could punch with the extend-o-arms, and use items, as well. The facial expression on the Clown Car can change when hit, etc.
His Final Smash could be dropping Big Steelies like the end of Super Mario World, or calling in Mecha-Bowser from Super Mario Sunshine, or Mega Hammer from Super Mario Galaxy 2. Heck, if they wanted to get really obscure, they could even use one of the mechs from Yoshi Safari on the SNES:
Then consider the fact that the original Smashboards rumor from a month ago said all of the Koopalings would be selectable as alts. The hit-boxes stay the same since the main fighter is the Clown Car. It'd just require slight new animations from the waist up to reflect the new pilot in control.
It's just a fun concept. I hope this all turns out to be real.