To me, the Koopa Clown Car idea patches my main criticism of Bowser Jr. completely. Bowser Jr. isn't very interesting without having some piece of technology to control.
Yes, his debut with the paintbrush was interesting, but looking back on it, he barely used his powers offensively. As Shadow Mario, he framed Mario for a crime rather than beating him up. Then all he does as Shadow Mario is run away from Mario, occasionally leaving behind a small trail of paint that could harm the pursuer. Then in the final boss battle, he fires Bullet Bills. The only other times Bowser Jr. uses his paintbrush are in the spin off games like Mario Power Tennis and Mario Hoops. The paintbrush was much cooler in the spin-off games, but because it lacked much impact in Sunshine, it probably never stuck in the main Mario games.
In New Super Mario Bros., Jr. is even less interesting. Just a weak fortress boss that has an uncreative koopa shell projectile.
Galaxy games? Never touches Mario directly, as far as I know. Just another nuisance.
Then in New Super Mario Bros. Wii and New Super Mario Bros. U, Jr. is a bit more fun to fight. He stays in his Koopa Clown Car and provides you with boss fights that are significantly more interesting than the Koopalings. And when the Clown Car gets boxing gloves in U, I liked him even more. So to me, Bowser Jr. using the Clown Car makes sense. It's what he's best at. It's what makes him more than a tiny, wimpy, Bowser-wannabe with a bratty attitude.
Sakurai, if the leak is true, kudos. You got this character right.