Rosalina wouldn't have been my first, second, or even third choice, but Nintendo has been making her more and more important since her debut. I think they have to be careful about which Mario characters to add because they've included all of the obvious ones. In fact, Sakurai and co. haven't added one since Brawl, unless we count Wario and even then, they made it a point to base him off the WarioWare series, similar to how Donkey Kong and Yoshi are but aren't Mario characters. None of the Mario characters I would have chosen over Rosalina have as big a role in the series as her so I think her inclusion is fair. A case can be made for Toad except every time he's playable, he's mimicking Mario. They wouldn't even give Luigi his vacuum so unfortunately, if Toad is essentially either Mario or faster-Mario, I feel like the chances of ever devoting an entire character slot to Toad is pretty slim. There's always Captain Toad, but he can't even jump. In terms of relevance, the only other Mario character I can think of with a reasonable chance is Bowser Jr. and honestly, I'd rather have Rosalina.