The old gods will only be a related thing but I feel like it is going more towards the void or "death" themed. Something beyond the old gods but related by being on that part of the spectrum. Void and Death stuff unrelated to old gods has played a large part that it makes me feel like they are a set up for the next expansion. Maybe something Ethereal related as well.
The defeat of the legion leaves a void to be filled. I'd just like to see more beyond "old gods". Would be interesting if Bwonsamdi turns out to be a different type of servant to the void lords. and we have a "Void Legion" so to speak.
Edit: Also I'd love to see a return to something in style with TBC expansion. I'd love that style of armor and stuff to be done with better models and graphics. I don't mind the current art styles but there was something I liked more about the more "cartoon" style of TBC (I don't even know if I mean cartoon but can't think of a different word)