Okay
.
How about for each of them..
For the Enji one, the villain is still the one pulling the strings and forcing him to act more babyish, but instead of Shouto taking care of his regressed father it’s somebody else; and in the end Enji is completely mentally and physically regressed.
Maybe with Shouto, he gets turned into a helpless toddler but is scared to go home, so he winds up getting himself taken in by Aizawa, who has to balance looking after him with trying to find out how to get Shouto back to normal?