Well they have already said that the expansion will feature an original raid. But I could see CT and ST becoming a riddle type questline around it or something to figure stuff out about the Allags and such. This reminds me the other day when I was in keeper friend goes... what's that light over there... and I look and go hmm... ahh that's the tower! So technically the tower is already part of the main story in that way. I guess it's one of those hints that it exists and that you can do it.