I imagine it being an AP bar filling up like FF. Certain people would fill up faster szince Ussop isn't that strong his AP bar would take two seconds, but Zoro's (I said it alright Tom! Myoti! Huh are you... just pretend I never wrote that) bar would take the longest. Because he would be the strongest main charecter in the game.
Maybe. But you may have gotten in a little... twisted, there. Usopp isn't quite as weak as you may think, though I think Usopp would be more interesting if his tactics involved using items. Like, he can mix some together to make new devices and perform his attacks (like Flame Star, Exploding Star, Caltrop Hell, and his moves using the Dials later on.) Oh, and yay on getting the name right, but he ain't the strongest... Luffy is.
I think that when you beat an enemy you would get a "costume" with their powers for a couple of turns like Mr. 3 and Mihawk.
Do wha...? O_o
Oh, I was thinking some more on the battle system. Well, technically more on Luffy's attacks right now (but I'm thinking each character should end up working a different way), so here's a few examples of how they might be pulled off ('*' denotes "Special" moves, available with enough... special... power... stuff >_> ) :
-GG (Gomu Gomu or Gum Gum; pick one ) Pistol :
This I see as his normal attack. He'll just jump up and stretch back. At the "height" of his stretch, a flash will appear on his fist, signaling to tap A (just a generic example) to shoot back, doing more damage than a normal GGPistol punch would.
-GG Whip :
Another normal attack, but it damages everyone (though less). Some sort of timing or such makes it work...
-*GG Bazooka :
Hold A to pull back then, when the signal appears, tap B to shoot back and do damage.
-*GG Axe :
Hold down on the analog (again, generic for now) until signalled, at which time you flick the stick forward, slamming Luffy's foot down (confuses and damages target).
-GG Bullet :
"Upgrade" to Pistol. When the signal on his arm appears, tap B the moment an "X" appears on the signal (think the 'O' on some One Piece logos), causing him to grab his arm and do more damage.
-GG Balloon :
Generic 'guard' move. Tapping A when hit (while using this) can bounce some attacks (projectile) back on opponents.
-GG Shield :
If not in Balloon mode, you can tap A when attacked to block, taking less damage.
-*GG Gattling :
Rapidly tap A to do damage (like you couldn't figure that out ). Before using, you can decided to focus on one target (more damage) or several (less damage, but to everyone).
-*GG Shotgun :
Luffy throws out a punch, then quickly flicking the analog back and forth (or up and down) causes more damage from the attack.
-*GG Buzzsaw :
Luffy grabs onto an opponent. Rapidly pressing a button builds up power (by spinning). When the meter reaches the top, pressing another button causes him to shoot forward and do damage (waiting to long causes the meter to fall, doing less damage).
-**GG Storm (probably like a "super special" or something...) :
Luffy begins by kicking the target into the air then puffing up (GGBalloon). Rotating the control builds up power, then tap A when the meter fills (kinda like GGBuzzaw). As Luffy flies up, another signal will appear, at which time you must rapidly tap A to do damage.
-*GG Fireworks
Luffy leaps into the air. You then must switch between tapping A and B to fill a meter (the letters change suddenly as the meter rises). When Luffy lands, he'll unleasd the move, doing damage to all opponents.
-GG Rifle
"Upgrade" to Pistol. As his arm goes back, rotating the analog causes his arm to twist up. Pressing A on the signal makes it fly forward and do more damage.
-GG Idiot (yes, I'm doing it XD ) :
Added defense move. Luffy has a 90% chance of dodging ALL attacks while using it(though he can't attack).
-*GG Octopus Fireworks
Takes a turn to get ready (when Luffy drops to the floor). When ready, a meter will appear (similar to the one for GGFireworks), then the attack will damage all opponents.
-GG Cannon :
Begins similar to GGGatling, but at the end will suddenly switch to the sequence for GGBazooka.
GEAR 2 moves will probably be the same, just stronger versions (kinda like going into trance, a la FFIX? ).