Wow, some megaton just dropped in today's announcement. For starters, defense maps, reinforcements are coming! Arena matchmaking will also receive an overhaul that should provide a better evaluation of team strength. It promises to take into the account of factors other than BST, like stars, levels, +x, skills. And even the BST element will now exclude the skill modifiers (Fury just becomes much less attractive now so I don't have to regret too much about having sent dupe Hinatas home previously).
Sounds promising and exciting overall, hope they can actually deliver all these in April.
Edit: Oh, they probably meant units fallen in earlier battles being unable to join in later part of battle series event, not actually perma-death in traditional FE sense, my bad.