I'd really enjoy more terrain interaction such as using spells, items, or abilities to get some extra tactical effect out of terrain besides just a small (or big, thanks FE5) avoid/def modifier. Some of this is already present in FE (snags, breaking walls/gates, light runes, etc.), but it's largely inconsequential due to barely affecting tactics (walls break very easily) or being incredibly rare or limited (light rune). Imagine if there were a light rune staff or other barrier-creating staff to wall off tiles or if you could burn forested/grassy/carpeted terrain and turn it into a dangerous, expanding damage zone or if you could freeze water tiles to create bridges or if you create an earthquake to weaken or destroy structures or create fissures or cave-ins and so on. Balancing all this to be a relevant part of a fun but still challenging and non-cheesable experience would probably be very difficult, of course, but if done right there could be a lot of value and uniqueness. Maybe it doesn't belong in FE, but terrain modification could bring a little more oomph to the tactics.
Berwick also has some really cool terrain-related stuff such as a skill that allows units to hide on building/forest tiles, allowing only foot units to cross house terrain, or even just the existence of cliff terrain at all which among other factors really elevates the gameplay above FE in many ways, especially the current state of FE.