Pauline? Yeah, I'm sorry, but I'd need a lot of convincing to believe she'd work in Smash. If you want a character to represent the classic arcade-era DK, I think Donkey Kong Junior is the best bet. He can pull not only from his own appearances, but from classic DK in general.
However, Pauline finally entering the spin-off realm and being in Mario Kart and the various sports titles would be great. More female variety that isn't another baby is always welcome.
However, and I know this is possibly controversial, but I want to see Mario's Cap Toss appear as a new Side Special for him. I feel like, despite the Cape being somewhat of a trademark of Smash Mario, there's just a lot of potential with the Cap Toss move. Throw it out as a long-standing projectile, works as an extra platform once, aim it at different angles to cover different recoveries...
I'd want to keep the Cape as a custom move, of course, but I also just want to flesh out the customization in general to be more like Palutena / Mii Fighters / at least Mega Man, but that's not too likely.