There is a Void champion that is probably slated to be released soon

Couple of hints towards this have been in the recent Targon story.

We know that their are many Aspects uniting at the same time because something is coming.

Aurelion Sol has also been called back because something is coming. In both Diana's and Aurelion Sol's lore, they fight against **Void **creatures, particularly, in ASol's lore he comments on there being a type of 'hive-mind' entity that is tearing a rift between dimensions and leading the void, he had to be called to stop it.

Not only is he being called again, but 4 other aspects have manifested at the same time. No other evil faction of Runaterra has been shown to offer resistance against Targon and ASol (Sol even comments on how everyone in runaterra is secretly 'conquered' by Targon and Shurima being the 'Premiere' civilization amongs humans, which likely puts them above the watchers).

So unless a new faction is going to be introduced to pose as threat, this only leaves the Void as a possible adversary to Targon and Aurelion.