Sargeras' spirit and body aren't one atm though. I think his spirit is still locked in Sargeras' tomb, isn't it? On the lost isles, where Gul'dan died. Perhaps it will start with the legion attacking Azeroth again, and we'd fight em on isles and places in the great and south seas. Azshara could be trying to free him from his tomb, with the trident taken from Neptulon during the Vasjh'ir questline (I think at least its hinted towards that). We defeat her and push the legion back, but sargeras is free.
I'm not sure where to find it, but a blue once said that we will definitely 'feel' Sargeras' presence looming over us, long before we will face him and the full force of the Legion.
After this I'd expect another Old God expansion before we take the fight to the Legion
Edit: I may be wrong about his spirit being trapped in the tomb though. It might have just been the Avatar defeated by Aegwynn. But Azshara freeing him in some way from somewhere might be plausible. Why else haven't he been able to take physical form, if he isn't trapped somewhere.