Why exactly do people think that Blizzard will create new capitals for existing race when they have gone out of their way to consolidate everything in Stormwind and Orgrimmar?
It's far more likely that Stormwind itself will be updated to reflect its new leadership (High King Turalyon and his advisor Alleria). The Vindicaar could hover above the city and Lightforged beacons might be set around the city for defensive purposes as well as a giant Light-infused dome ala Velen's Shield to protect the city from any kind of attack.
The Ren'dorei could reinforce their presence within the city, for instance by setting up twisted mazes and corridors to disorient any potential invader or criminal, as well as maybe reinforce the Stockades with Void enhancements.
Since Alleria has now reached the upper echelons of Alliance leadership, I expect the Ren'dorei to gain greater foothold in Stormwind.