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.
If I'm understanding your question correctly; you'd use a skill bar. Your character would likely take a stance once combat starts based on your chosen Bending/Non Bending form. Then, you'd chose skills that relate to that element/style.
Let's say you choose a Waterbender; in combat you'd go into your stance then select a skill like "Water Whip".
I'm not an expert on MMOs so, if anyone else has better recommendations; go ahead.
maybe I should have been more clear. I want to know more about how combat would work. Like for example, turn-based or real-time? Does dodging simply take the form of an accuracy debuff or do people need to actually get out of the way to not be hit?
I'd guess it'd be real time combat. Although, it could be turned based due to how Bending animations could work. I'm not a programmer.
I'd love to see the character customziation though! Choosing your Faction/Bending style, Hair, Face, Clothes, maybe Lore/Backstory, etc. That would be fun! Maybe you could choose advanced choices like being a Sun Warrior or Swamp Tribe. Although, those might just be unlockable later in the story?