Actually, that gave me a few ideas! I know it's hard to give advice when it's still very nebulous. The Chaos Sorcerer is prime for being made to look like a WoW lich, like Amal'thazad.
I've been looking at the Blight Kings, I think they're the perfect Abomination addition.
Someone had told me I could use skeletons as marauders in local play, I would just have to get a specific base for them and let my opponent know what they are. I've also had my eyes opened to kitbashing, which I had no idea was a thing! There are some really cool Harbingers of Decay that have been kitbashed.
Belakor could super easily be added in, so I'm looking at him too. Man, so many choices! And if I end up slowly accumulating all of these over time, I'll have things to swap and try out!