I feel that the Mario RPGs are merely serviceable in this day and age, unless they make a new Mario RPG that involves one of the other major Mario characters in a significant manner. Original characters are fine, but making a familiar Mario character a major presence in an RPG satisfies fans of the characters and even adds a breath of fresh air to the RPGs through the addition of never-before-seen characters (in an RPG, anyway).
What I mean is, Mario has a wide cast of characters, but the RPGs tend to only focus on the three or four main characters: Mario, Peach, and Bowser (+ Luigi). It's rare that Yoshi even gets involved, even if he got his turn (thank you, Mario + Rabbids). The partner characters in Paper Mario don't fill the void because well, they've never gotten much involvement beyond the game that they were introduced, have they? The same logic applies to Mallow and Geno, who only made sense in the game that they are introduced in (though to be fair, Mallow is more open for a future appearance compared to Geno despite the former's unpopularity).
Partners in Time is an example of what I mean as a positive change: sure it's just Baby Mario and Baby Luigi, but both of them have been involved in a Mario game before and so it's nice to see established characters go on a significant adventure. It's your favourite characters doing some grand stuff. Donkey Kong never even got this chance, because his DLC campaign in Mario + Rabbids is a separate adventure that doesn't involve teaming up with Mario. So in this case, a Mario and Donkey Kong teamwork RPG game has great potential, as the movie even shows that a team-up between both works spectacularly.
Likewise, an RPG that involves some lesser-used major characters like Daisy, Wario, and Waluigi (and maybe Toadette and Birdo, but they had the benefit of appearing in an RPG so it's less pressing for them) would be such a treat to me. I admit that on Paper Mario: The Origami King's feedback survey, I even mentioned that I would like to see Daisy appear as a major player in a future Paper Mario game. (To be clear, major player might even just be a non-player role like Peach, just make her have a non-negligible role, unlike Luigi in Sticker Star)
Maybe you might see me listing Daisy and Waluigi as "major characters" and do a double take here. However, I stand by my stance that both of them are major Mario characters. This leads to another unpopular opinion of mine, where I view them with a similar level of prominence as what others would consider as major like Wario and Rosalina. For one, on
Nintendo's character portals, both of them have their own profiles, which is not something you see for just about any character (not even Kamek has a profile). Also, the Mario series is not known for canonity, so it's not like how Legend of Zelda dictates which characters are "canon" based on the games they star in; by saying that say, Waluigi, doesn't count because he's not in the main game, it's reinforcing the view that the main games are more canon than spinoffs and thus doesn't align with the lack of canon adherence in the Mario series, when Waluigi is more prominent than many characters in a Super Mario platformer due to his frequent appearances. Of course certain games don't get referenced often like Mario Party, but Mario Kart and sometimes Dr. Mario are plentifully referenced so it's not like spinoffs are off-limits.
(On the note of Boo in the character portal: since Boo is not established as having THE Boo in the same way Yoshi and Toad have THE Yoshi and THE Toad, Boo can't be considered a character as much as it is a species)
Thank you for reading.