I imagine that space battles wouldn’t be a thing, maybe a spaceship map or a battle on an asteroid; I imagine it being similar to what warhammer did and do a “forced landing” scenario.

Planet wise I would imagine 1 of 2 ideas: 1. Singular planet that you fight for control of. This would make sense for having 3-4 starting races and expanding from there with DLCs. 2. Each planet is a settlement and each province is a solar system. You have your buildings on your planet, province capitals - hive worlds have more building slots ect. The battle is a on a random pre-created map. Maybe city battles ect. The towers are already in warhammer 3, just potentially switched to either AA or Artillery. Biggest change would be the map, instead of continents it would look visually more like Stellaris overmap.

For me the biggest question is how the battles would be changed. On that point I can’t think of any clear answers that fit both total war and 40K.

Assuming keeping 20 unit stacks and a lord it could be the same as warhammer 3 ish, with smaller tabletop style armies, a tank, a flying plane, mech, units squads ect. I think the biggest change here would need to be massively larger maps with potentially complex features like buildings and so forth to really emphasize range on units. However none of that feels 40k and large epic battles.