I hope if Bowser is one of the playable characters that he is like Donkey Kong in the first game's DLC in that he has unique abilities which change the way the fights works pretty substantially (not the same abilities as DK though). Would be a good way to mix up the gameplay if he is a mid-to-late game team addition.
Also, Rabbid Rosalina basically being a moody teenager is great. Reminds me of this scene:
Also, Rabbid Rosalina basically being a moody teenager is great. Reminds me of this scene: