I think Mera Mera is already really good, but some changes could be added. Here are my suggestions:

Hiken
Already perfect imo, but the hitbox should be adjusted so it doesn't hit multiple times, or if it does, adjust the damage to scale properly.

Higan
Also work fine as it is rn, but maybe making it shoot the projectiles from both hands so it looks more similar to the anime.

Dai Enkai: Entei
This move should have a second activation input before being launch instead of inmediatly being shot. An increased size will be nice and some circular particles around the player while charging it up. I don't know if the hitbox has been changed, but it also need to be bigger and more accurate with the fire ball.

Hotarubi: Hidaruma
Instead of the fireflies hitting the objective and then burning them, they should make a little explosion when getting near the target and any entity hit by those explosions gets the burning damaged.

Jujika
I really love the cross-shaped projectile, but the ability could be change to a charge up one (really short timer), sending a light beam (like Pika's or Goro's ones) before sending the actual project at a very high speed. It also needs a damage increase, since it's really low rn.

Enkai: Hibashira
Instead of just burning the nearby enities in should also inflict damage and knockback upwards, so it likes more like an explosive attack to keep away your enemies sending them flying and burning them.

Kyokaen
Give it kind of an spiral effect like in the anime and make it destroy projectile attacks, making the ability staying in the same place during its duration (since we didn't saw Ace redirectionating it). Also, if an entity or player is in the hitbox of it, they receive burning and knockback (maybe damage, but kinda low). If it just faces on one direction, any attack that comes from anywhere else can damage the user.

Heat Dash
I really like that Mera doesn't have a flight ability and I want it to stay like the dash we already have, but maybe changing the particles to come from the bottom part of the user's body will look better, something like what Ace did while using his boat on Arabasta or the way Sabo moves during Dressrosa, where their legs turned completely into flames. Other way I think could look cool is making just the head of the user visible and replace the rest of the body with fire particles, like Sabo when he fought Fujitora. The name can be change to Otebisha, which is a technique Sabo used recently in the manga and it's practically the same as Heat Dash in the mod.

MOVES THAT COULD BE ADDED

Enjomo
Yeah, the return of Enjomo, but now it creates a circular wall (with a decent radius) of fire particles around the user that prevents other entities or player from getting away knocking them inside any time they want to leave and burning them for a short period of times. This ablity has a long duration an cooldown, the wall will be kinda high, but players can escape flying or jumping out since it won't have a roof.

Shinka: Shiranui
Basically two lance-shaped projectiles that impales the target, they first impact and stick to your enemy, eventually the lances "explode" causing burning damage (I don't really now about this option since it's just another projectile attack, but it could be a nice addition idk).

Moeru Ryusoken: Kaen Ryuo
To be honest, I don't like it to be just a normal Mera Mera ability. What I was thinking is using it like a Brawler ability but that can just be access if the player consumed Mera Mera no Mi, you know, as a requirement. It would work like Roukugan, the players need to hit the opponent, then they grab him before sending him flying in a rotating flame attack. Instead of doing a huge amount of damage, maybe it can just bypass armor, so the damage it inflicts is always the same, and of course the knockback and burn.