bring the squats back!
They're called Demiurg (sp?) now I think. Whether they'll ever have an army I don't know. There are a few other races that also have potential in 40k, like the Hrudd and Enslavers but I'm not sure how you'd go around putting them in. The most important consideration is going to be the fluff though, and how these things fit into the galaxy.

Also, for fantasy, I'd like to see the other humans get their army lists, would be cool.
Fantasy is tricky, as there's already something of an over-abundance of armies as it is (even after the Dogs of War and Chaos Dwarfs got dropped in 6th edition). The problem this generates with human armies is that, while fun fluff-wise, it's very difficult to create a distinct and effect playstyle for them that doesn't encroach on another race's niche. Particularly given the versatility of Empire armies.
On a tangent, I do remember seeing one really impressively converted army where the guy had created a Cathayan army using Brettonian rules. Don't know if it's online anywhere these days.