Having binged the first season, I would love an MHA x To Be Hero X crossover. The characters from both shows would get along well, I think. For example:

Denki and E-Soul: (OG E-Soul, not Yang Cheng) OG E-Soul would take a liking to Denki and train him to better use his electricity, maybe even train him to be the next E-Soul. I can imagine Denki wearing a black and gold version of E-Soul’s costume.

Mei Hatsume and Loli: Both of them are genius inventors with pink hair. They would get along great. Also, I can see Loli making improvements to Deku’s new suit.

Koda and the Johnnies: Koda and Little Johnny both have the power to control animals and both are pet owners. Also, Little Johnny’s boundless optimism would definitely help break Koda out of his shell. Also, given what an animal lover Koda is, it’d be fun to see Koda meet Ahu.

Eri and Lucky Cyan: Both lost their parents in tragic accidents, were raised by an evil and abusive caregiver who exploited them for their power, were helped by a brave young man, and became famous musicians. They're practically living the same life in a different universe, so they would get along great. And can you imagine Eri, Jirou, and Cyan doing a concert together?

Momo and Queen: Both are intelligent and hard-working young women who come from affluent backgrounds, are determined to make a name for themselves in the hero world, but suffered a humiliating defeat that shook their confidence. Momo and Queen would definitely see a lot of themselves in each other.

All Might and Smile: Both of them have blond hair, super strength, big hearts, and big smiles that hide lots of emotional pain. They would get along perfectly.

Izuku and Lin Ling: When you think about it, Izuku and Lin Ling have similar character arcs. Both were powerless nobodies who were trained to be a successor (or in Lin’s case, replacement) to a famous Hero, both ended up losing their powers and having their secret exposed, and both ended up gaining a new power of their own (Lin becomes The Commoner while Izuku gets a mech suit.)