Oh, I'm not sure about that, Wario does a lot of heinous things in the WarioWare cut-scenes.I feel like he'd be a good pull for a Halloween event, he reminds me of a cross between Stitch and Jack the Clown. If we're gonna get technical, Wario lives in "Diamond City" and drools over money. As for Rosalina, I'd buy that she's busy tending to the planets, though I feel like she could be a Christmas character. I imagine she might appear for the Lunar Festival in Japan, that to me makes the most sense. I mean. Any character from the Mario franchise could appear in the parks in terms of "story" (as has been said, Mario and Baby Mario race against each other, though I DO think they'll not do this sort of double exposure in the parks, I think they will stick to some integrity). I'm just saying that this is story that a character attendant might tell a disappointed guest.
At the end of the day, these four are absolutely the correct characters to put on as meet-and-greets. While some might want to meet Bowser, I think that would necessitate an extra turret extension onto the Bowser's Castle set piece to have Bowser be a puppet on a throne for the same reasons Donkey and Ariel can't move. But I'd rather have the Bower Jr. Boss Battle.
I also want to shout out to Joe. Excellent post.