Time travel is cool. The caverns of time instances are really fun and let players experience the lore from back then. But, that's all they are is short little stories that don't advance the over all plot at all. They're wonderful little tid bits for lore buffs as a small part of an expansion but as a whole expansion it's just a waste of time.
There's a few reasons that I can see why if an entire expansion would be based around time travel it wouldn't be good:
1. Predictability. When the LotR movies came out everyone and their mother's had already read the LotR books and knew what was going to happen. While they were fun to watch everyone knew what was going to happen before the first movie was even released. In the same manner if you consider the story of WoW to be an ongoing process a time travel expansion would be like hitting pause on the main story and going and doing some random stuff that you already know how it's going to end.
2. It doesn't advance the story. Similar to the butterfly effect in the past we can't change anything. Every bad guy whose going to die has already died. Pandaria was a big leap in that it contained relatively no lore from the previous warcraft universe except for 1 character during a small portion of 1 game. But, Pandaria still advanced the warcraft story. It continued on Wrathion's part in the over all story and has set up the burning legion in a huge way. But, if we go back in time we can't actually continue on this path and won't actually see anything new.
3. It's a huge cop out. Players already complain about rehashed game play. A time travel expansion is just that. It's Blizzard reusing a bunch of old crap instead of actually progressing the story of warcraft.
4. Nothing matters. Unless it's stop the infinite dragon flight (again been done before would be boring again) then nothing that we accomplish in the past actually matters. All we'd be doing is playing a scripted role similar to puppets in a play where everything ends up the same every time. It's not like we're stopping kil'jaeden from entering azeroth. Or killing the lich king. Or stopping deathwing from fully destroying the planet.
5. Time travel just doesn't logically make sense. Bad guys can't disrupt the time line and destroy time because then their previous self dies and then their future self isn't around to disrupt the time line and so on. Good guy's can't go back and alter stuff because then the even never occurred that made them want to go back and alter it so they never go back and alter it in the first place.
6. They'd have to pull a new race/class out of their ass that has nothing to do with the current content. So far every time we've had a new race or class new added to the game it's been current content and is relevant to the story. If they make an expansion about hopping into a portal and going back in time then we can't meet up with a new race/relevant class since how would that race/class interact with modern azeroth?
Really when it comes down to it time travel/caverns of time type stuff are a fun way to see the old lore. But, really they're only good for an addition to the main story line as a side quest rather than the entire focus of the expansion. I have no problem with revisiting Draenor to some new area that was unknown or perhaps go through a portal to an area where people fled to during the destruction of Draenor as long as whatever we do is current content and not stuff done in the past.