[ArchDragon]

Spoiler for My prediction::

Miss Maria and Yukariko will either die or get severely injured during the attack, and will be replaced by those 2 girls we see in the new OP.

Reason:
From the convo between the Corals about Nao's ascension to pillarhood, we know that only position IV is vacant, which I assume was the one offered to Mai.
So for those 2 new girls to become pillars, 2 more vacancies has to be made.