X-Men 3 held a casting call looking for "ODD & UNUSUAL multi-racial men, women and children of all ages". I'm going to go out on a limb and guess that this means we could see the Morlocks in this episode. If you’re not familiar with the Morlock, the name comes straight from H.G. Wells' Time Machine novel were an underground race attacks the humans living on the surface. The Morlocks in the X-Men Universe are a group of deformed mutants that hide in New York's sewer system form the hate of the regular people on the surface.
Will we see a X-Men 4? That really depends on the success of this film and the ability to reorganize the cast. What we'll probably see next will be Wolverine released in 2007 and if we are luck X4 in 2009. David Benioff, writer of Troy is working on the screenplay and Hugh Jackman is signed to lead the spin-off film.