So void lords appear in the prequel to Legion, where they grab khadgers legs as he enters tomb of sagares. It doubt that is the only / last thing we hear about them - so possible something for next expansion.
We also encounter old god stuff during many artifact quests, and also during this pre event as we enter Ulduar.
Queen Azshara shows up in aszuna during our quests there looking for the pillars of creation.
If anything Legion shows us a lot of enemies, that can be possible expansion or plots leading to ... Something.
Who knows if LK will use the situation to pull some strings and attack us while we are fighting the Legion ?
Exodar is also getting ready to leave azeroth, which can lead to something.
As I see it there are so much that depends on the content patches coming. Lots of possible plots that can lead to interesting stories and adventures.
Personally i would like to see some more old god stuff , something dark and evil