Personally, I don't think Warlords of Draenor and what it's said to be would make for much of a WoW expansion. I am however hoping it'll be a new RTS for a few reasons:
1) The plot/timeline (the one where we go back in time) people are suggesting lends itself very well to set up a prequel to Warcraft I. Obviously it wouldn't be about going back in time, but setting it in that era works.
2) A time-travel expansion (some of the leading speculation suggests we'd be going back to pre-Outland Draenor) would be very hard to do right. It works okay for dungeons and content in small portions, but a whole expansion might be a bit much, given the multitude of ways we could affect or alter the timeline during our adventure.
3) There's been a lot of fan demand/interest for a Warcraft IV, and in a way, this could deliver. Maybe not exactly Warcrat IV, but think of it more as 'Warcraft Zero', kinda like a 'before the beginning' deal.
4) Mainly my opinion, but I think there should still be a bit of time before we have a Legion war again. They don't know how to reach us, and Wrathion implied we have some time to prepare. We still have some corners of Azeroth left to explore (Kul Tiras, Zandalar, Kezan, Ny'Alotha, etc) and threats that still need to be vanquished (Queen Azshara, N'Zoth, King Rastakhan, Zul, etc). Save the Legion stuff for later, and I'm positive it'd be well worth the wait.
5) The 20th anniversary of Warcraft: Orcs & Humans (aka: Warcraft I) is coming up next year and releasing a brand new Warcraft RTS that explores some of the pre-history of the game's lore would be an awesome way to celebrate!
I'm certainly open for a Legion-based expansion (possible playable Ogres? hell yeah!), but I feel the time just isn't quite right. Warlords of Draenor, I think, would be far more interesting as a new entry into the Warcraft RTS series. It's been a while since Warcraft III. A new one would be great, especially with so much pre-history to Draenor that has yet to be expanded upon in the games. I know it's been in books, but this'd be a great chance to live it and celebrate the game that started it all!