Hi here is a few ideas.

Add Asmodeus to be included in the gods clerics can follow. He's a god in 5e. With evil clerics, give them the option to use negative energy so to learn Inflict Wounds instead of Cure. Since there are evil gods available, for the paladin class have an option for an anti-paladin or blackguard. Also add a dragon blood sorcerer as subclass for sorcerers. Here's an idea let players become vampires and assuming their class still works with it they keep their levels - like for instance an evil cleric or sorcerer would still keep their other powers but a Druid or ranger would not as undead are affronts to nature. Since evil clerics can use negative energy from my first idea you can heal your undead characters. Also add an encounter with a Lich and make him ridiculously creepy. I think a touch of the genteel or sophistication juxtaposed with the Lich being what it is would be an interesting combo.