If done right - would it be awesome to have an Avatar MMO? There are several possiblities if you go that route for stories, characters, locations, themes etc.
For the Era: We have many options. Pre-Aang's, (*Edit* Not Aang's), Korra's, Post Koora's.
I'm thinking that the Player could use between about 4-5 "Factions". Each having their own Bending forms plus about 2 or more special Bending forms. (Non Benders would have specialized skills)
- Earth Kingdom (Earthbender) (Specials - Metalbending or Lavabending) (Sandbending could be used if Pre-Aang)
- Fire Nation (Firebender) (Specials - Lightningbending or Combustionbending)
- Air Nomads (Airbender) (Specials - Spiritbending (Air) or Soundbending *New*)
- Water Tribe (Waterbender) (Specials - Spiritbending (Water) or Plantbending) (Bloodbending could be a story related form)
- Non Bender/Republic City (Nonbender) (Specials - Weapons, Gadgets, or Pet.)
I'll get into potential starting locations, potential threats, potential quests, potential characters etc. later if anyone wants. (May add other *new* special Bending if needed. Might even get into potential roles/classes if needed.