Oh yeah, adding Levi, Ramuh and Shiva would be the dream but i don't think they'll give us that though. At least not on this expansion.

It would make a lot of sense to eventually have an Astral/Umbral phase. i like that.

In an older thread I even pitched how each of those three new summons could work in order to feel different form the 3 existing gems. However, if at this point in time or in this expansion, the idea is just to add ONE more summon only, the pairing of 1 gem and 1 demi seems reasonable and dynamic enough to me if we are to have the 5 existing summons plus another one (NOT Solar Bahamut). The demis should feel "earned" in some way instead of being openers followed by the gems. That feels backwards.