Please add Bahamut as a summonable avatar.
I understand that the reasoning behind not adding him until this point was to keep his image as something that seemed important, or special, or untouchable. Now that the precedent has been set by Alexander and Odin for "Astral Flow"-only summons, I think adding Bahamut as a summonable avatar should be reconsidered. Bahamut could be another "Astral Flow"-only avatar. My thoughts are that Bahamut would be summoned by targeting an enemy (like Odin), then Bahamut would use his signature move, an AOE Teraflare, after which you would lose your Astral Flow buff, just like Alexander and Odin. Because Ifrit already provides a fire based aoe Astral Flow blood pact, I'd suggest making Teraflare a non-elemental attack, like meteor. I think that balance-wise, this would be acceptable, as Bahamut couldn't replace the Astral Flow abilities of the other avatars in every situation, because you're able to use the Astral Flow blood pact of the other avatars (Titan, Ifrit, etc) 3 times, and you'd only be able to use Bahamut's once before the Astral Flow buff wears off.
Please consider adding Bahamut as an avatar
. Bahamut is an iconic image in Final Fantasy, and I know a lot of summoners were disappointed to know that he wouldn't be summonable.