This is definitely something I've been hoping to see for awhile. Here's my input and I hope it helps.
I very much like the idea of having to build into a bending master. I think it would be cool if there were seperate "levels" so to speak. For example everyone would start as a beginner, however just like in the show, there could be a random chance to start as a prodigy, like Azula. Now I understand people will say my idea is OP but it is again for an RPG type server. So, without further adieu, here's my breakdown.
Beginner - could only use one or two basic abilities of their bending type. Must be actively bending for an hour total and have killed and or healed 25 entities to be advance.
Abilities are weaker in every way at this stage, (.25 full potential) after meeting necessary requirements, become a Beginner 2.
Beginner Prodigy - work extremely similarly to beginner, but abilities are stronger (.5 full potential)
Beginner 2 - no new abilities, abilities are simply strengthened. (.35 full potential)
Prodigy 2 - (.65 full potential)
Must be using bending again for an hour and same killing / healing reqs.
Beginner 3 - (.5 full potential)
Prodigy 3 - (.8 full potential)
Same reqs to advance, to next stage.
Next you would become an apprentice, this is where beginners and prodigy beginners converge to even out.
Apprentice - learn 1 or 2 new moves, staying at just .85 full ability potential throughout. Must use bending for 2 hours and heal and or kill 50 entities in each stage, apprentice 1 through 3 to advance. In each advancement the apprentice would learn a few more abilities.
After reaching all of these reqs you would move on to a "realized bender" here is where things get tricky. There would be between 5 and 10 stages involved here and this is where you could begin learning sub-bending. I'm not really sure of the reqs to move up through the ranks here, so we'll move on. After completing all stages, you would become a "master"
At master, you would have all abilities and sub bending abilities. And the most powerful abilities, far exceeding the regular, base power.
Just an idea and not complete yet! Again this is for an RPG server, the kind of people who want to play like the show. Thanks for reading!