Well, Raphael is definitely one of the most interesting characters of the entire game (and the best villain imo).
I'd love more Raphael content, dlc or update, I'm definitely up for it. Honestly, I think some ideas on the list should be in the game already... such as the option to visit the House of Hope if you sign the deal with him and that victory dinner he promised us.

If we could get more dialogs and new scenes with him it'd be absolutely amazing, but making Raphael join of our team like Halsin or Minthara it's a big NO, it would only ruin his character and I'd never want that.

Although I can see room for flirting dialogues and a romance/fling with him. As a fling I see it being just like Mizora's, while as a romance it'd probably be a very toxic one that ends terribly bad, with MC losing their soul or becoming an asset in his conquest of the Hells.