Decay is Tomura's own Quirk, one that he was supposedly born with, and this allows him to "decay" anything he touches with his hands. This essentially means that what he's touching will crack apart and crumble away to dust, whether it's living, organic matter or inorganic. Tomura is able to control how far and deep the decay spreads, and if he wants, decay can be spread to multiple targets as long as there's a point of contact for its effects to travel from one to another. For example, it can travel through the ground and hit people who are standing in its area of effect; likewise, someone who chops off an affected limb can prevent Decay from reaching the rest of their body. At this canon point, Decay is very powerful and was shown to be capable of leveling a small city.
For the purposes of the game, I'm willing to have Decay be restrained by the resort: I'd like it if he could still affect fairly large structures (walls, doors, etc.) but perhaps have it unable to progress more than a few meters. I'm also cool with it if the resort would like to shut it off completely in certain locations or circumstances, so that there are places and things he can't Decay at all. I'd also like to have it still affect living beings, with player permission if it involves a PC of course.
The other Quirk he obtained through surgery is
All For One, a copy of his mentor's Quirk and all its stockpiled resources. All For One allows him to steal and grant other Quirks completely and permanently: if a Quirk is stolen, its original owner can no longer use it, and if a Quirk is granted, the new owner will be able to freely use it. This applies to All For One's user too, which means Tomura and his Sensei have access to any and all Quirks they've stolen. Because Quirks are sustained in the MHA universe by world-building aspects, my assumption is that All For One won't be able to steal other abilities or powers that aren't Quirks, though it probably would be able to grant a Quirk to someone without one. When applicable, any stealing/granting would be negotiated with respective players.
Because All For One has a ton of stockpiled Quirks, see the
wiki for an exhaustive list. Below are ones I think might be more relevant in RP.
+ Regeneration. What it says on the tin, basically allows him to heal from damage inflicted on his body. This can include severe maiming and burning, though there might be a limit to it if his head was chopped off or if he was burned to ash, but this is currently not concretely known.
+ Air Cannon. Can fire large blasts of pressurized air from the palms of his hands.
+ Search. Allows the user to "search" for up to 100 individuals at one time, as long as they've previously met. This Quirk will show the user the locations of their targets up to miles away.
+ Radio Waves. Allows him to jam radio waves.
+ Rivet Stab. Allows him to summon and wield thin, black blades of unknown matter. They are not free-floating—they come out from his hands and spine, but can grow or lengthen and change direction in the air.
Finally, in the same surgery as above, Tomura was granted superhuman abilities in speed and strength, so he can do stuff like punch really hard or jump really high, etc. It was also stated that the introduction of hundreds of new Quirks has essentially caused his body to go haywire, leading to some pretty gnarly situations where flesh and bone have grown uncontrollably, though it seems he was able to control it later on. I don't know if anyone would
want it, but I've considered keeping this possibility open for spicy purposes, but on the day-to-day, the resort can help him keep it together physically.
Most importantly,
I've been given mod permission to use All For One to give Quirks to other characters. Since All For One has a large stockpile of Quirks (which aren't always shown in canon until they're used), the ones that are given away will likely be made up. Tomura
would not do this extremely freely, and would not get rid of any Quirks that he finds useful. There will need to be a lot of IC relationship development, then I can talk to players OOC-ly about the powers their character might want and negotiate what they might be like.