I'd suggest considering who is in possession of those much-speculated-on 'keys'. Louisoix's staff was the 'key' of Thaliak was it not?

Alisae & Alphinaud have now made several mentions of their possessing another 'key' which they plan to use in some form. Assuming it's of the same nature as the one their grandfather used I would say they most likely will embody Nald'thal.

Beyond that I don't believe we know of any others currently within the Scions control.

It's worth pointing out that I don't believe the archons actually are the embodiment of any of the Twelve. We know from the 1.0 storylines that these keys empower summoning rituals in some manner so it would not be too much of a stretch to suggest the archons merely have the means to summon one of the Twelve or somehow access their power.