Luffy really wasn't that satisfying, but he does have a lot of potential. A lot of his attacks could do more damage overall. His jump and dash attacks could easily have twice the damage. Luffy's elasticity isn't really captured in this game in my opinion. He doesn't have the movements that reflect the sudden jerkiness that is rubber.
I'll again bring up his Rifle hitting multiple times, which seems completely unintentional with how much damage it can do. The combo only needs a bit of tweaking (or maybe adjust hurt and dizzy lengths) to make everything feel better to connect. Doesn't have to be guaranteed, but it shouldn't leave him open. Lastly, why can't he hit during the middle of Pistol? Feels awkward having to wait to hit someone from a distance.
I was going to say Gatling Gun has poor damage, but remembered how you can extend it by mashing A. It's *almost* broken. At critical health, Amidori does nothing to lower your MP when you follow it up with GG in the corner (it's inescapable unless you're a ninja.). That's close to 400 damage for a full bar of MP; the attack does close to 250 minimum from a full bar and full health. Granted, you won't have all that MP in a real fight, but in a close fight a well-placed Amidori can easily give Luffy the victory (characters like Edward cannot escape even in the open field; fix that please). Going on a second test turns out I can do over 250 damage to Zoro and well over 400 in critical health. Check to make sure his falling bdy frames are not bigger than everyone else's. I'd really like to know why this happens, because it's not fair for combos to work differently unless they have different health or defense.
Armament GG doesn't really do much more damage, and only serves to do damage faster from what it seems. There are two solutions to this: Raise the damage, or lower the cost. Making it do more damage overall would make the move have a real use. There really is no point in using it right now as the opponent will simply have more health to get back instead of taking the time to get your opponent in a less favorable position. At critical health, Armament is obsolete because of the regular version's lower cost allowing for it to hit more. Seeing how Elephant Gatling does about the same damage as Armament up close, make the same adjustments as Armament (don't alter the projectile part). It's riskier to use from a distance, but it's the only other good followup from Amidori aside from Conquerer's.
Conquerer's Haki CAN be hit out of during startup, unlike what A-Man posted. I'm concerned about raising the damage seeing how A-Man is fine with it while I think it's a little low. I also feel like the MP cost is rather hefty. It's not spammable in my opinion. I actually like how it can break every healing item. Definitely a dominating move, but be sure not to use type 3 so players can't use it to exploit AI.
Elephant Gun could use a better hitbox around his body to give him better reach and allow him to follow up from closer distances. It misses a lot if you're too close. It only needs to be on the first frame where Luffy extends fully so he covers right in front of him. Gigant Pistol is a much more useful option overall because it hits all around him.
Lastly, make Luffy only able to use Observation Haki when guarding or dizzy. I used this to escape several projectiles using its z-movement and made many attacks that rely on multiple hits useless. I find it better than the ninja's subsitution because I had better control of where I was going.
For characters like Luffy and Hibari, you can make the attacks that extend have a shorter recovery by removing a few frames after the full extension so it doesn't look like one long reversible animation, like what was done with Pistoru.
On the subject of rowing: it's a little too good unless you're Edward. Then it's horrible. I advise lowering the horizontal momentum a good bit because when you're real high up it creates a gap that only projectiles can cover. At least cap the initial momentum instead of letting it last the entire time in the air. Players can go as far as half a screen.