I personally don't see Rosalina showing up in Paper Mario, at least not to a very noteworthy degree. Lumas are more likely though.
The reason I think this is because of Koizumi's strictness with the character (and to a lesser degree, Miyamoto's apprehensiveness to the character).
You'll notice that EVERY appearance Rosalina has made since Galaxy 2 has been completely without story context (or at least her appearance specifically is, like her being a post-game unlockable in 3D World for no real explained reason in the story. Captain Toad: Treasure Tracker didn't even elaborate on it either, despite it being a prequel to 3D World). People often say that "Nintendo doesn't care about the integrity of the character's story because they put her in everything", due to the "she only comes near the Mushroom Kingdom every 100 years" plot point in Galaxy. However, Nintendo have actually NEVER broken this rule. Rosalina only ever appears without context, and usually in scenarios where every other character does too. Games like Mario Kart, Mario Party, Smash Bros, etc, have no story or real context for the characters being there. Everyone is just there because they are ensemble games that let you play as these characters for the sake of it.
However, this is not the case with the RPGs (both Paper Mario and Mario & Luigi). The RPGs have stories, and every character has a context for being there. As such, they can't (or at least, shouldn't) break that rule they set for Rosalina. While Rosalina had context for being there in Galaxy 2, Mario went out into deep space himself and ran into her, rather than her going near the Mushroom Kingdom. While they could theoretically do this just fine in an RPG game, that means they have to have a good reason for Mario to go to deep space to begin with. Which I see as an unlikely scenario (Super Paper Mario did it but unfortunately, that game started development long before Rosalina was a thing).
So yeah, I don't think Rosalina has a possible role as a major, plot-important, or even side, character in any RPG game due to the circumstances of her character.
At most, I can see:
-Her showing up at the climax of the game provided the game builds up to something like the fate of the universe hanging in the balance, and is a crucial part of saving the day since she's effectively god in the Mario universe.
-Her being the host of some kind of post game content, like an arena of super bosses linking to "warriors and monsters from across the cosmos", with her possibly being the final super boss herself.
-Brief cameo or mention as part of a random gag.
-Appearing as one of her Cosmic Spirits to aid Mario in some fashion.
-Non-canon amiibo functionality, similar to the Fire Emblem characters in Codename STEAM.
Granted, I think of the two RPG series, I think Paper Mario is more likely to utilise her due to the fact the Paper Mario universe is more lenient with relevance to the main series (i.e. it takes more liberties, such as there being good Koopas and Goombas). But I'm not counting on it. And hey, that's fine, I don't need her to appear in everything.