Just an attempt on my part to explain several aspects of how I see the tactics of combat working in this system I'm putting together. Those of you who've been following my work on d20 Rethought will find alot of familiar material here, and as such the same will be true to a lesser degree for anyone who's familiar with the d20 system in general.
Item 1: Character position and range
I'm thinking that this rpg won't invest a great deal of effort into tracking character's locations respective to one another. A rough grid of ten-foot square, where characters that are in close combat share a single square, sounds like it ought to be workable. A long-range duel would be a little less dangerous because participants are investing some of their skill into making their attacks long-range enough to reach the enemy, rather than into making the attacks deal more damage. Meanwhile, a melee-range bending duel could be made to match the fast pace seen in the show if you make Buildup actions (see below) easy to disrupt in hand-to-hand combat and thus give characters a reason to focus on quick back-and-forth moves.
Item 2: Action Types
All characters can take one Primary, one Secondary, and one Side action per turn. You can give up a Primary Action in favor of another Secondary action, and give up a Secondary action in favor of another Side action. Some actions will be labeled as "Immediate" (i.e. "Immediate Secondary Action). This means that you can take them at any point, even if it's not your currently your turn. If you've already used up that action for the current turn, you can still take an immediate action by expending your use of that action on your *next* turn.
-Most attacks and/or uses of bending are a Primary action. Same goes for most combat actions that require a skill check.
-A "Buildup" check is a secondary action. This is where you make a skill check, divide the result by five (rounding down), and add that as a bonus to your subsequent primary action. Possible buildups include an Acrobatics check to swing from the chandelier prior to making a flying kick or using earthbending to lift up a particularly huge boulder before you hurl it at someone. Alot of offensive bending moves will work best if you use a buildup action first, essentially allowing you to dedicate the results of two checks towards the same single attack and thus give that attack better odds of breaking through the enemy's defense.
-A defense check is an immediate secondary action. Normally, any skill check made against you- to attack you directly, sneak by you, deceive you, whatever- will compare the opponent's d10 roll+skill+modifiers against your skill+modifiers+5 (unless I change the die mechanic, but that's a matter for another post). A defense check lets you roll too, and use that result (if better) the next time you'd use your passive defense. It can also sometimes let you use different skills to defend.
(What all this means is that during most combat rounds you'll make a single attack, but must carefully decide just when and how you'll make your move- do I use my secondary action to Buildup my own attack or Defend against my opponent's move?)
-Skill checks that can be multitasked with relatively little effort (say, making a Manipulation check to lie to someone during a fight) can be done as a Side action, but carry a -2 penalty because you're not giving the task your full attention. You can't multitask an attack, that's always a Primary action.
Note that Buildup only lets you use a check as a boost to another check. You don't get the normal benefit. So if I actually need to make an Acrobatics check before I'm close enough to my opponent to kick them, I could do the acrobatics check as a Side action (with the -2 penalty) and also spend a Secondary action to make another Acrobatics check as buildup for my attack.
Item #3: Initative
During combat, your Initiative Count- i.e. the results of your initiative check- determines who goes first. When combat starts, everyone who's aware that it was about to start gets a free initiative check; otherwise, your count starts at 0, which means you have to spend a secondary action to reroll your initiative before you can do anything else.
I'm considering two different wrinkles I could add to this process. The first is having init checks be an *immediate* secondary action, meaning that you can try to act quickly and outdo your opponent that way. Remember Toph's first "fight" in the series? Against The Boulder?
Yeah, kinda like that.
The second wrinkle is to allow various things to modify someone's iniative count. Used as a cost for a bending technique, this would give me some nice leeway- and I'd definitely have it apply to anyone who gets a solid hit scored on them in combat, forcing them to "get their bearings" again (i.e. reroll initiative) before they can get back in the fight.
Item #4: Bending Faliures
Remember the time Toph tried to make a huge dirt cloud with Earthbending, then flubbed it and had to try again? No, because if someone screws up a bending attempt in Avatar it's either due to outside interference like being punched in the face, unexpected issues such as a solar eclipse, or them being an utter beginner. So what happens when you roll poorly on a bending check?
One of two things. Option A is to accept the poor result, which usually just means a relatively small-scale bending move. Option B is that the move takes longer. You take a Secondary action to reroll the check with a -1 penalty, and can use that result instead. This can go on for multiple turns so long as your character does nothing else and can keep concentrating on their bending, though the penalty on the check increases by 1 each time.
All right, I think that's that for now. As always, feedback is welcome; let me know if everything I'm describing makes sense, and whether or not it sounds like something you'd want to play.