Unfortunately, Blizzard has given the idea short shrift (ha!) to say the least -- while there are lots of rumored expansion plans, even beyond Wrath of the Lich King, it's mostly the Maelstrom and the Emerald Dream; nothing about the Leper Gnomes in Gnomer. You'd think they wouldn't be that hard to clear out (lower level groups regularly make short work of them -- ha!), although radiation is tough to fight off. Even the best Gnomish scientists seem to hold the belief they'll be able return to Gnomer, so it must be possible, right?
But to make a long story short (ha! OK I'll stop), retaking Gnomeregan just isn't on Blizzard's priority list. The city might work great as a portal hub, or even a new setting for a BG or Arena, but as it is right now, Gnomeregan is staying in the hands of the Lepers and the Troggs.