Based on the recent Famitsu article the battle system takes a lot of notes from SO3 with some slight improvements/alterations, which is cool to me as I feel it has the best battle system of all the games. Personally I would have liked to see MP-death return too, but that's just me, it seems to have been pretty hated by a lot of people.
Off the top of my head Shining Resonance and Tales of Zestiria don't transition to separate battlefields. SO5 appears to have made one of the better overall attempts at this however.
Yeah I guess it's good that there's some kind of excuse, but it still feels pretty flimsy and transparent (pun not intended). Traditionally the runes are red and clearly visible, so I don't know why hers don't seem to be noticeable until they glow or whatever. My only guess is that they're actually all over her body or something, which makes me fear there'll be a scene where they all glow or something and the camera just has to pan all over her body to show them, but maybe they'll show some restraint (lol).