Ren'dorei should be reworked or removed from the game and why in the actual hell would their capital city (which they will never have) be in Quel'thalas? I think that would be a bad choice. However, I could accept this as a possibility:
Ghostlands is revamped, the northern section is purified as OP mentioned and features a new questline in which Lady Liadrin leads the Blood Knights to clean up all the way down to Deatholme. However, once the Dead Scar is healed and the Blood Knights reach the gates, it is decided upon that the Forsaken, who have provided aid in the Ghostlands from the start, will be gifted Deatholme and the lands surrounding it as a new home. The San'layn, should they be introduced as an AR, may join them here. The new Forsaken territory would create a buffer between Quel'thalas and the Plaguelands, help solidify the Horde's hold on northern EK, as well as bring the Amani insurgency to heel, whether the tribe is destroyed and the survivors flee south or they are permanently relegated to Zul'aman. Of course this would be no easy feat, given the tenacity of the forest trolls. Such a struggle could form the crux of the new Ghostlands storyline, perhaps involving worshippers of Hakkar? Or is Hakkar perma-dead...idk I don't play SL.
Either way, don't let the Ren'dorei have anything besides their tentacle asteroid.