This about it like this, When warcraft 1 and warcraft 2 were made, how tied into warcraft 3 where the storylines?
When warcraft 3 was made, they made several thousands of new charactors and had them reference all the classical charactors from the original game.
It's very quite possible for there to be a new Warcraft RTS, but Blizzard likes to do things new and innovated, so first they would have to develop a new concept for the game to run on, then afterwards they could create new charactors to fill in whichever part of the storyline they would like end WoW over.
Or if they really wanna go insane, they could have the new warcraft act as a prequel+sequel, rushing through the classic games and WoW's storyline up into a new storyline. But really what blizzard would have to do is make the next warcraft rts new.
They cant just make it another warcraft 3 expansion.