This is a controversial one, but I would really like some more story content for Quintillus.
I feel like the reason that a lot of people dislike him, is that he is a setup without resolution, while most other characters are fun from the get-go.
I think that his involvement with Tollus and his Demon Cult, and the reactions to the player's questioning that he has, as well as some questionable mannerisms have a crazy amount of potential for character development. He has a very tight connection to a story arc that is integral from the very start of the game, he has the whole statue mechanic, he obviously keeps a lot of secrets that the player would like to know.
But because of his initially cold demeanor, and his exclusively dommy/demeaning sex scenes, with no real, implemented options for character development/ befriending, a lot of players skip him entirely because they consider him flat out mean, and particularly uninterested in the player. (and miss out on some of his exclusive companion content, which... I love his snide remarks and would like to see more of! )
I'd love to see Quintillus' character evolve to be befriendable at the very least, maybe with a trust/ respect mechanic similar to Ryn's confidence, that actually evolves his relationship with the player and makes him a little more, for lack of a better term, palatable to the players that currently dislike him. Cait has a lot of feminine BFF energy that I think Quin could be a really fun counterpart to. I really liked him at the start in any case, and I think the character has a lot of unused potential.