Snoke had another apprentice?

Force Ghosts, son. Rian Johnson has actually indicated that Luke being 'unlimited' in that state is very well a part of the plot for Episode IX.

There's no way that they aren't doing this. Luke's the biggest nostalgia-factor that they have left. ("Member Chewbacca Again?" or a supporting role for Lando ain't gonna cut it.)

 

I think the thing is that Snoke's "other apprentice" only exists for the purpose of establishing that Kylo Ren was the best student that Snoke ever taught in the background lore. He or she won't be coming into play in Episode IX, as they are most likely already dead. Now, for stories set before the Sequel Trilogy, I can see this being different. This character or these characters, I imagine, will be the primary threat to some kind of before-everything-goes-to-crap-for-our-heroes story arc in the years between the Battle of Jakku and Leia being outed as Darth Vader's daughter. Rest assured that they will explore that avenue eventually.

As of right now, the Sequel Trilogy will most likely finally get around to explaining just who the Knights of Ren actually are and establish them as a new threat that the heroes have to deal with, because Phasma is probably down for the count (unless we get another "she actually survived somehow" moment) and Hux is now a joke (though I think that The Last Jedi kind of set him up to be someone that Finn will ultimately defeat, since he's responsible for the entire conditioning program that he had to suffer through and this would be a nice way of wrapping up his story).