Considering we SMNs don't really use stats for anything I don't see why Atomos would have that ability nor why it'd need to be locked to Astral Flow.
As it is I can see Cait Sith (assuming she is an avatar) and possibly Atomos as normal avatars.
As an expansion of the "new" summoning way I can see Phoenix with Rebirth Flame ( fire damage to nearby enemies and AoE Raise effect with removal of exp penalty, probably casted on dead players) at level 85 and Bahamut with his Megaflare at 99. Bahamut though should break the 9999 damage cap that Odin has on all enemies and not based on HP%.
I kinda like the way this could go because we'd be able to get lots of amazing abilities, even if only through Astral Flow.
Cait Sith, if she is an avatar, could be a new Carbuncle. I'd love to run around with her every once in a while. Just add Cait Sith boots as a reward somewhere that halve her perp. cost

Bahamut is the easiest to implement considering we already have a quest with his whisper as reward, they'll just need to make him available and reduce his size a bit.
Knowing SE you'll probably have to get the whisper to access a new battlefield though, hopefully a solo battle against the ultimate avatar. That would be sweet.
Keep locking the new avatars to levels please SE, I was extremely happy when I found out that Alexander and Odin can only be summoned at 75. Gives us something to look forward to other than merit BPs.
We should probably speculate on which abilities our current avatars will get before anything else lol. I can see a Healing Ruby III in the horizon but I'm hoping for more buffs as well.
Diabolos has Cacodemonia still locked and Fenrir has an animation for something that even Fenrir Prime can't use. But other than that they'll have to come up with completely new animation and abilities which is interesting. We may see something starting with June.