Next Demi-Primal?

[Iscah]

Coil is also a side story and they bent themselves backwards to retcon a reason in for Phoenix to exist.

Depends whether they started from "we're doing this; come up with a reason" or "we can justify this in lore, so let's do it".

Phoenix has a ready-made reason; the same one that they already used to allow summoners to use Dreadwyrm Trance. Alexander is harder to justify because it hasn't been throwing its aether everywhere, but also because its actual powers don't become clear until late in that story arc. Compared to a dragon or phoenix, where you already know what to expect from them.