I think that Fenrir and Siren should have their own primal fights since they were reduced to simple dungeon bosses it's really a shame since some lesser summons got a real primal fight like Bismarck and Lakshmi. This would also expand their lore, maybe they have followers in other regions of Eorzea that have heard their terrorizing tales and have come to worship them as gods. Or the Ascians resusicate them and empower them to cause havoc in Ishgard and Limsa Lominsa.