I think the second & third movies should serve to introduce the rest of the core cast that appears in all the spinoffs tbh. Not all have to have major roles, just establish that they exist. That way the post credits scenes can be saved for characters/plotlines truly nobody predicted or expected to appear. That is my completely not biased at all take. Conversely, I expect them to dripfeed us the main cast over 5+ movies and never go too far beyond that. Rosalina is one that I'd prefer being introduced in the second movie.
I genuinely expected Yoshi to have at least a minor role in the first movie since the trailers showed a herd of them. And Lumalee made me expect explicit Galaxy ties out the gate including Rosalina. So definitely explore that more in the second film. In addition to Yoshi, I hope the second film introduces a handful out of Rosalina, Wario, Waluigi, Bowser Jr. or Daisy (but please not as a love interest for Luigi). I want to hope for Toadsworth as well, but I think sadly the first movie was his only chance and it was missed.
Third movie can introduce any of those who didn't appear in the second, establish a larger role for Pauline, and then in the post-credits my dream would be a tease of one of the RPG plots. Preferably one from before 2012. And then we can have versions of those stories that do involve more of Mario's main cast, which was something a lot of people didn't like about the RPGs. I know it's a pipe dream but goddamnit i never want to give up hope on that. Imagine a canon connection between Rosalina and the Star Spirits or Geno.
All in all, I don't want them to spend the entirety of the movies they make dripfeeding us the main cast, but sadly that's what I think is most likely.