Verticality + Flight might be more complicated than just a flatter system, maybe flight could be programmed to be relative to a surface but that'd be clunky.
Then again there is height in BG3 but right now only Ladders and Jumping to specific surfaces allow one to get higher, though perhaps they could program flight to have a button that allows you to move 5 up each time (1 movement) and then they can select to move as normal for remaining movement and 5 down? That also might be finnicky and could have issues of rising through ceiling or floor meaning there would need to be programmed a ceiling for every map that is also at a height that makes sense (shorter in a building, higher outside).