I wouldn't mind Rosalina taking over the galaxy games.
It's no news that Nintendo is trying (and in some parts succeeded) to break the Mario franchise into many different sub-franchises.
Yoshi once in a while has his own Island game and appears rarely in Mario platformers (I think New Super Mario Bros. Wii and WiiU are the only ones since Sunshine).
Wario has his Wario Ware / Game & Wario shtick, he never appeared in any major platformers and at this point he probably never will.
Donkey Kong is now Donkey Kong Country and nothing else, I think he isn't even playable in games like Mario Party anymore.
Luigi hopefully is detaching himself into Luigi's Mansion which is a great franchise (Luigi's Mansion WiiU please), but I guess he'll always be playable in platformers and Mario & Luigi games.
They even tried to do something for Peach (Super Princess Peach) but I guess it didn't work. Maybe they're trying something like that with Rosalina but I wouldn't understand why didn't they simply created a new franchise instead of pushing a character from a known franchise. Well that's Nintendo.
I hope Toad / Captain Toad will have their own eShop line of games based on the minigame in Mario 3D World, that was brilliant. I can't see it become a full-fledged retail product though.