Singer calls it an inbetweenquel. So that in a sense is a sequel to both. As for character development in the future, there can't really be much as the events will be wiped from canon once the past is changed. But it should at least give us a good idea of what the characters are a like a decade or later after X3. We'll get to see what grown-up Rogue, Iceman, Kitty and Colossus are like. Their persona's would still be very similar in the altered future. I'm sure whatever interpersonal relationships issues there are will also somehow transfer to the altered timeline. Beyond that I doubt we will see big arcs on each given none of it really matters in the end.