Patch 10.2.5, Seeds of Renewal, will include a Gilneas revamp and reclamation. While this questline isn't ready for testing yet, we can speculate on how this story may unfold.
Gilneas is a human kingdom located southwest of Silverpine Forest. When the kingdoms of Lordaeron were overrun with the undead plague, King Greymane of Gilneas took desperate measures to protect his kingdom, barricading it off from the outside world with a Great Wall, and authorizing the use of a «secret weapon» — releasing worgen from the Emerald Dream to attack the Scourge. However, the worgen curse was not easily contained, and before long it began to spread throughout Gilneas.
Around the start of the Cataclysm, Gilneas went through several upheavals, all at the same time. First, the Cataclysm itself badly damaged the kingdom, with parts of the Gilneas peninsula falling away into the sea entirely. Second, Garrosh Hellscream, Warchief of the Horde, had commanded the Forsaken to take Gilneas. Under the ruthless leadership of Sylvanas Windrunner, the Forsaken assault was swift and deadly, and King Greymane's own son was killed by Sylvanas during the conflict. Finally, the worgen infection had spread more than ever — Even Greymane was revealed to be worgen.
In the end, Gilneas rejoined the Alliance. Driven out of their home, the people of Gilneas sought refuge in Darnassus. Though the Alliance managed to retake Gilneas itself, the kingdom has remained largely abandoned — partly due to the ongoing threat of hostile Forsaken neighbors in Silverpine, which only ramped up during the Fourth War.
Players have been asking about the return to Gilneas for ages, and we were promised that an event this important would happen in-game.