I've been thinking about the Mario X Shantae RPG idea some more,and I wondered, what if it could be a sort of successor to Mario + Rabbids, by using the high fantasy setting suggested by Seven Sirens? Both sides fighting in a way they are not used to, yet mysteriously have the knowledge for anyway.
The playable heroes on the Mario side being the familiar Mario, Luigi, Peach and Yoshi, accompanied by Shantae, Bolo, Sky and Rottytops, in the following roles:
Mario and Shantae, much like the Dragon Quest heroes use a healthy mix of physical and magical combat + support. Both can use shields with one-handed weapon (mace/small hammer for Mario, scimitar for Shantae), or a two-handed weapon for extra power (big hammer for Mario, magic scepter for Shantae). Mario is more physically inclined while Shantae is more magical.
Bolo and Luigi are the physical powerhouses, but in different ways. Bolo lacks any magic whatsoever but has very high physical stats, wields powerful flails and can throw objects like bolo balls carried in your inventory for even more damage. Luigi meanwhile is a sneaky rogue who is very squishy, but also very good at keeping aggro off himself, debuffing enemies and landing strong critical backstabs.
Peach is the main healer with defensive support spells, while Sky delivers the offensive buffs while also being able to fight in the air and from a distance depending on how she's using Wrench (as a mount or portable attack bird).
For Yoshi I decided on something unusual in that he's actually a martial artist, forgoing gear in favor of more gains by leveling. He can also be used as a mount by another party member for extra agility. Finally, Rottytops is, by virtue of being undead, the tankiest member with huge amounts of HP, drawing aggro, having her own specialized healing items and remaining somewhat active when knocked out.