If I had to choose a Mario RPG character to introduce into the main canon, it'd probably be Prince Peasley. Adding a prince alongside the princesses seems like such an obvious thing for the Mario universe to do, and there isn't much lore you have to get to understand the character conceptually like with say Fawful or Geno - there's a bean country next to the Mushroom Kingdom, it's enhabited by beans instead of mushrooms, extremely simple to grasp.
I'm personally partial to Vivian and she is a character who a return of is demanded, but I feel her design is so abstract (especially in regards to whether she's a ghost or not) that she could be offputting to casual players even if the entire fandom, casual and hardcore, miraculously agreed that having a trans character is OK. Lady Bow is a runner up, she's easily my favourite of the enemy-based RPG party members and would be super easy to sell casual players on ("existing character but girl" is a tried and tested trope, as is "gothic + feminine aesthetic")
I'm personally partial to Vivian and she is a character who a return of is demanded, but I feel her design is so abstract (especially in regards to whether she's a ghost or not) that she could be offputting to casual players even if the entire fandom, casual and hardcore, miraculously agreed that having a trans character is OK. Lady Bow is a runner up, she's easily my favourite of the enemy-based RPG party members and would be super easy to sell casual players on ("existing character but girl" is a tried and tested trope, as is "gothic + feminine aesthetic")