I've heard this idea many times before, but I don't know that we've ever officially covered it here on the site: there is a fairly large movement in the player base, especially among the shorter folks, to have Blizzard continue the Gnomeregan story line -- and give Gnomer back to the Gnomes. Gnomer is pretty much the most hated of all the instances (well, Uldaman might barely overtake it in some circles), and as happy as the Gnomes are in Ironforge, a lot of the pint-sized ankle-pokers would like to have their city back.
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.