i spekulate we may have to fight zodiark in the future butt i also think we may have to fight hydalin as well

tho for primals i think alexander is a iconic holy element with phoenix and for the dark element maybe doomtrain

also eden would be a weird fight since its only 'real' appearance was in ff8 ( ff13 it talked to people using statues it moved so i do not know if we can call those statues part of eden or eden itself)