Preface: I never played Fire Emblem, but a lot of turn-based strategy games and many roguelites. So fundamentally I believe in your idea and am very excited about such a game. Now let me dump way too much of my own opinions and ideas on the genre in this post that might miss completely what you have in mind. Maybe you can still take some value from it.
In games like AoW:PF and Into the Breach, I enjoy the game from the first battle, because immediately there is a small riddle. The starting units are not 100% straight-forward; there are 1-5 neat tricks to discover. It also depends on the race/troupe chosen. I prefer to play those that have more tricks and less damage. In West Marches, to my taste, there is not enough of this yet. I hope you will add a lot more variation and wonky abilities and so on.
I played a lot of Planetfall recently. There, I really enjoyed the kind of battle where almost everything dies and both sides fight to the last man standing. Unfortunately those are usually stupid to take, because you lose so much. Similar in ItB: the game is made in such a way that you should mostly ace every battle (which is always possible), otherwise you lose eventually. Same in FtL. Same currently in West Marches. This is a genre trope of tactical roguelikes: Autistically min-max losses vs. ressource gain every battle, at the start, so you can beat the later stages. I wish there was a game that gave more leeway on the losses you take, and made encounters more tense to win at all.
Small notes:
- Mouse controls are really unintuitive, and partly broken (wheel doesn’t work in overworld)
- I think in my first game I started with 4 archers, which is certainly funny in a way but was confusing since I needed a few turns to understand how to attack
- In the encounters where you rescue someone, I think the overworld text was pretty good and concise, but then in the mission you had to do "recruit" again and then there was some weird text. Why don’t those guys just start out as recruited already?