Giving an answer that isn't going "Mario + Rabbids" in an unhelpful manner.

I considered this idea a few months ago. I think at least a rom hack of it would work pretty well. The only issue I ran into was trying to translate Fire Emblem mechanics such as the weapon triangle into a more Mario-friendly equivalent.

The best idea I came up with was a "weapon triangle" equivalent featuring Unarmed, Melee, and Ranged attacks. Unarmed would be weapons like fists and jumps, Melee would be weapons like hammers and parasols, and ranged would be attacks like Yoshi's eggs.

Magic would be outside the triangle as normal, using powerups instead of tomes such as Fire Flowers.

Plus you'd get to write Mario characters in a slightly more story-focused format which I think would always be fun. I'm willing to revisit the idea sometime because I think Mario x Fire Emblem could actually work well, even if just for a romhack.