i feel like they're setting us up for a time skip arc
ryne, gaia, alphinaud, alisaie, galool ja, maybe even y'shtola are going to grow older (hagcat)

i think there's the matter of thancred's aethersickness that urianger is helping him with, so we might see ninbreaker again.
zero also has a chance to reappear in pld form.

we already have a time skip method with travel between worlds requiring time dilation and time sync, and the key is in our possession. there's also the tycoon underneath the crystal tower if the key seems too stable.