This is a well put together theory that I'd like to thank @GROUNDZERO- for (All rights to them for the evidence):

Evidence:

The League of Villains need highly advanced technology to create Nomu monsters, which is something Hatsume uses while making her "Babies". During the sports festival (An event that was on TV), she used Tenya to model her inventions. What if the League found a way to get in contact with Mei and because she is very proud of her creations she would have accepted the deal to help make technology for the villains so that she could show off her creations. The Nomu would also then be her creations (or "Babies"), meaning her creations would be all over the news, making them well know and famous. Hatsume isn't a protagonist or main character, it would make the viewers schocked that it was her, since it s was never to be suspected.

Opinion:

I believe that this is a well thought out theory that would make for a great twist in Boku No Hero, this is now one of my favourite theories.

Rating:

4 Nomu out of 5. 🐲🐲🐲🐲