Of course it wouldn’t be a sub series like DK, Yoshi, Wario and Captain Toad have, it would be like Luigi’s Mansion and Princess Peach: Showtime. Who knows though on what kind of games Rosalina could have, we know for certain that they in some way would make use of space and expand upon some things about our beloved Guardian of the Lumas: https://www.change.org/p/let-s-give-rosalina-her-own-game-series