Diverse Map Conditions
What I meant by this is those map conditions like in Path of Radiance or Conquest Chapter 10. The defense maps were one of the most funnest and challenging, I would love to see them make a return. I have also heard that Thracia's condition was great too with the "Escape all". I would love to see them as well. Having to rout the enemy commander or kill all of them remains but sprinkles in with different map conditions more please and not just the paralogue in 3Houses.
Fixing Choose your path
Honestly I would say that 3Houses was great with this. However, with the first twelve chapters being mostly the same. I would prefer that they would reduce the total amount of paths or have it mostly different. It was a chore on like the third playthrough and so on. However on the other hand it promotes repeated playthrough. What i would suggest is just to reduce the amount of paths available from 4 paths to maybe 2 similar to alm and celica on SoV which both having different route and not having the same paths but sometimes crossing with each other.
Shrink the Monastery / More functions within camp
Either shrink the base so it doesnt feel that cumbersome, or you add more functions within the camp. Right now we have a huge amount of space but it felt like paddling. However I know that some people enjoy talking to various people within the Monastery so I would suggest that they would add more things to do. I don't have ideas for that unfortunately.
Now i really love Fodlan's world building. This isn't a improvement box per say but I really absolutely love this aspect in three houses. Showing off different continents connected to the world. Having potential allies and members from different part of the world in various continents were aspects that I really enjoy reading in three houses. Having tidbits of said continents to read for in the library was an added touch. I would love the care of world building to go a step beyond or remain the love and care in the next series. My one gripe is the world containing fodlan is unnamed. So maybe the next one could name the world too 😛
Class variety in three houses was a flaw in my take. For one, I really like that we get to choose on what classes we want but i also prefer the predertemined route that older fire emblem have (though I feel that this system would be sticking, it leave a good foundation for future games). I would suggest to have more variety so it wont be all samey