Let me start with this: I enjoy both Mystery Dungeon and Fire Emblem, Greatly.
I need a Fire Emblem Mystery Dungeon game, developed by Spike Chunsoft and Intelligent Systems.
Honestly there is not much I feel I need to say about this one.

Magical towns, charming characters and good, snappy, responsive controls.

Looking forward to reading your feedback below.

I'd rather play Federation force.

This hurts me greatly.

Instead we should get Tokyo Mirage Sessions #FE Second Note. A Sequel on Switch.

But aren't both games already grid based combat?

I know the gameplay between the two is significantly different, but it still feels pointless.

They are both grid based combat, but yeah they play very differently.
Mystery dungeon has some really good towns with charming characters.
I think, with FE's gameplay philosophies: Mystery Dungeon could be taken to the next level.

Both series can benefit from the crossover, resulting in a fantastic game.

I don't think FE fans would like it. It's too simple

A Warhammer 40k Space Hulk crossover with Mystery Dungeon would work better.

Problem: Niche already handled by Gaiden and the Gaiden remake that will be released soon.
I know those two games simply aren't what you mean, but they are technically dungeon-crawlers in a loose sense.

So make a bad Fire Emblem game on purpose?