How do people think it might happen?
Gnome king finally decided to send someone high enough level to kill some level 20 Troggs?
Dwarves finally got sick of gnomes and helped them retake gnomer to get rid of them?
Gnomes built a new capital and just named it Gnomer?
More phasing in the game = complete gnomer instance and get the city back, with option to redo old instance (perhaps a stockade type instance?)
Personally I think it'd be cool if the Cataclysm shattered the land and separated Gnomer from the mainland, also causing the Trogg's to be cut off from their reinforcements, finally allowing the Gnomes a chance to retake their home city. This might also explain why (if at all) Gnomer becomes a new city for Alliance, as it wouldn't just be right next to IF any more. Perhaps portals from IronForge gnome district might become available to port to it??