Elenarie: The small isles where the tomb of Sargeras is were raised from the water floor. The rest were always there.
Crosmando: Where's your evidence for this? in WC3 TFT Maiev arrives at the Broken Isles and says that they were "recently" raised from the sea floor, that WC3 mission takes place around the ruins of Suramar.
Also the Tomb of Sargaras was destroyed in those missions, you need to escape as it's collapsing. So using it in the new expansion would be another retcon.
Forget the WC3 maps, they were never made with the whole world being visible in mind. The actual world, not just those isles there, looks much different than what was shown in WC3. Also, you have to take into account the cataclysm, it very much changed the location of all isles. Which is why you see Tol Barad in the northern parts of Eastern Kingdoms, although it originally was located near Kul Tiras, south, which we still haven't seen.
The tomb wasn't destroyed, we didn't see the end of what happened, we just ran away from there. It is Warcraft, never assume anything. :D
At the end of MoP we got the War Crimes book which tied together the end of MoP and the beginning of WoD, and explained much of the original timeline / alternative timeline thing. It is likely that we will see another book before the release of Legion to explain us how exactly Gul'dan knew where to find Illidan and the tomb.
Which leads me to think, is this the alternative Gul'dan? It has already been established that demons cannot die outside the Twisting Nether, and once bound to the Twisting Nether, all of their "alternatives" stop existing. Thus, what if Gul'dan back when he was killed was already a demon, meaning the one on Draenor is the one Gul'dan that already invaded Azeroth in earlier years?