While not an MMO at all, in fact not an MO even, have you played through Kingdom Come Deliverance? I thoroughly enjoyed it, as did the Mrs, and it provided a decent challenge and a some great stories. If you like graphics it also looks really decent as well, doesn't bother me but I know some people will care.
I can't think of any MMO's at all, if you still have the itch when WoW: Shadowlands comes out maybe worth ago since you'll be at the same state as everyone else. As for WoW Classic TBC yes it would be amazing, I think it was my favourite of all the expansions as it still had good challenges, a good story, and offered an open progression system with all content available to everyone from the day it launched other than the very final Sunwell patch.