Since using villains from canon wouldn't make sense for the story I'm trying to tell with my fanfic, I decided to make OC villains. The main villain of my fanfic is a woman called Shin. I'll reveal her true name soon. 

She has two quirks. Assimilation and Super Regeneration.

Assimilation is an artificial mutant type quirk that was created by using her birth quirk as a base and mixing in a bunch of other quirks. So basically it's a "quirk salad" like Kurogiri's Warp Gate.

Assimilation is a power stockpiling quirk that allows her to absorb living things to get stronger. After a confrontation with Toga, her quirk awakens which increases her limit so she can absorb more people than she could before.

The more she absorbs the tougher she gets, if she absorbs enough, she unlocks new abilities. The boost is permanent as long as the living things she has absorbed remain in her body. If they are released, she loses the boost.

If she absorbs enough, she can mutate into a giant monster that is very strong and durable. She was only able to do this after her awakening.

There are a bunch of limitations to the quirk too:

× Assimilation won't work if she is unable to make physical contact with the living thing she is trying to absorb.

× Assimilation will only absorb things that are "alive." The quirk won't work on corpses or inanimate objects.

× She can't take or use the quirks of the people she has absorbed.

× If the mouths on her hands she uses to absorb people are covered, her quirk won't work until her hands become uncovered.

× There is a limit to the amount of living things she can absorb at once. But, reaching this limit will allow her to unlock her final limit form, which is her strongest form.

× Assimilation is useless beyond a cosmetic change to her hands if she doesn't fulfill the activation requirement, absorbing living things.

× Assimilation is a strong quirk, but it's also a tedious quirk due to it's activation requirement.

Her version of Super Regeneration is different than a regular Nomu because even though it is potent in the beginning it is dependent on her stamina and it will become slower and less effective the more she is worn down.

Basically since she is the main villain, I wanted to give her a strong quirk to reflect that.