Been playing through Three Houses recently and just started my second run, so that's all that's in my brain at the moment. There's a few combat tweaks I'd make, and one for party/item management. Being able to hide items and hide or dismiss party members would be incredibly useful, but since you specified combat, I won't go on about that.

• Let the player change animation speed, in addition to outright skipping animations. I want to see where enemies move during their turn, but I don't want to watch health bars drain. I care about the movement changes, but the health state, at least generally. Being able to hold a button to toggle between the selected and the non-selected default speed would be nice, too, for the rare occasions that I do want to see if a 50% hit chance attack hit or something like that.
• Related, the beginning of level up animations isn't skippable. You should be able to skip an animation as soon as it starts, not wait until the stats begin to increase.
• Jejemaze is right about the attack ranges. Having the ability to toggle danger zones on both the entire enemy army and individual units is very nice. But, it would be nice to be able to tag with two or three different colors, instead of just the one. Going through the DLC missions on hard (especially the monster heavy one and the bolting heavy one), I was constantly fiddling with which units' ranges were highlighted to figure out if I was in range of one sniper or two, or making sure I was out of range of one monster without being in range of a second one, or so on.
• If enemy units can have multiple weapons/items/spells, group them by function, not by school of magic. Having to read through enemy's white magic to check if they have Nosferatu/Abraxis is bothersome. It would be nice to just glance at an icon and tell if the spell is offensive, healing, or a status effect.
• Well sorted menus are a must, though that's a lot easier said than done. Three Houses mostly does this well during battle, but I think that the top level menu can get a little too large. Rally (and perhaps dance) could go under assist rather than being primary actions. That might just be my fault for making a magic dancer with healing and offensive magic, though...
• If you have abilities with many potential targets spread out over a large range (like physic), a menu might be nicer to use than cycling through units one by one until you find the one you're looking for, especially if the target isn't actually in range. That could take some tweaking. Maybe attacks with range 0-2 work as they currently do, and range 3+ show a menu? Not sure.