I would like to see a power that gives more AC. Divine Armor- gives avatars +5 AC, +10, or +15 depending on their status. This may be a bit high with a greater god in plate armor and shield now would have a 35AC. This could be +3,6,9 if you want, but I see most of the avatars not wearing plate mail, but more with no armor, or just light armor and this just compensates for the lack of plate.
The other thing I can see each avatar having is the ability to summon reinforcements. I see the legendary action to summon some sea creatures, but I was thinking of some sort of angel/demon soldiers. Being a legendary action means that they can be summoned each round. If this is the case, I can see the summoned be weaker than if the power was 1/rest or 1/day power, even recharge. A hoard of CR 1/2 can still hit the PCs while being wiped out each round and CR3 soldiers can take a few hits. I was thinking of CR3,6,9 depending on the status of the god.
I like the idea you are presenting and I look forward to seeing more. I have never had any of my groups fight an avatar, but maybe some of the evil gods would be handy to have.