You can fuck Alma in two instances. Both require you having a moderately high Magic stat and charming her with her own spell. In fact, Alma has the longest scenes that allow you to screw her.
You can fuck the Demon King if your Charisma is high enough and you choose the right line of complimenting her tail. You can also do it in battle if you're strong enough.
I don't think Dark Knight will ever get a scene as your bottom. She's a 100% dom character.
As for other characters, their bottom scenes are being added slowly. For example, recently you got the opportunity to top Minotauress, and in the Story Mode you can top the Jester.
This type of content is not the most popular kind among the audience. People don't feel the urge to give it to the enemies, as much as getting dominated and topped. So I doubt Majalis will be doing hero in active role arts in the future. Because doing new arts for bottom hero earns more interest.