I have a few suggestions:
1.) Maybe a configurable percent chance of an element to be assigned to a player on first join
(Including chi)
2.) A configurable percent chance for said element to come with sub elements
(Ex: Earth has a low chance of being capable of lava bending, fire has a low chance of combustion, water has high/max chance for ice and moderate chance for plantbending, etc.)
3.) Each move can drain a configurable amount of stamina/hunger
4.) A system which could claim territory for a nation/city that works with the plugin
(Similar to Factions, Towny, or anything of the like but more bending-friendly)
5.) A bending level system that makes it to where the more you use an element the higher level you get, increasing the damage you do by a configurable amount, decreasing cooldown by a configurable percent, gaining a configurable amount of skillpoints, and more.
(The elements would all be seperate from eachother and have different levels. Giving people with multiple elements a reason to use both equally)
6.) Ability to use scrolls/visit bending masters to be able to learn new moves using bending skillpoints
7.) Have a configurable chance that the comet appears for a configurable amount of time
8.) Having mobs that could use bending/wandering npcs that can bend
(Could work with citizens and such)
9.) A whole set of new foods that come from the show, all with configurable properties such as saturation, hunger fill, and effects
(Would also be great if default foods were able to be edited as well)
10.) This one's a long shot, but maybe give a set chance that the sub element will be assigned after an amount of time
(For example a player would join and be assigned earth, they play on the server and after a while (a time to time set in the config) the sub element that was assigned to them at the beginning by chance (lavabending) would kick in, giving them the ability(s) that come with that sub-element)
TLDR: After being assigned a sub element on join have it to where it doesn't work until a certain amount of time has passed while playing on the server, the player would be unaware of the sub-element until the time is up
I know most of these are over-complicated but I think they'd add a little more depth, if I need to explain more just say so, I know I can be a little vague ._.
