Originally Posted by X-Maniac
That's a possibility. It then means, of course, that the time traveller and the present-day X-Men will then need to travel back to the 60s. That double time-jump could work. But remember that if it's between the events of X2 and X3, then Jean Grey is apparently dead (in fact, cocooned beneath Alkali Lake).
I don't imagine all of the original X-Men will return in DOFP..but if you wanted Jean Grey in the movie then the traveler would end up between X1 and X2.
If you were referring to the bit about Rachel Summers, I'm pretty sure Jean was "dead" in the comics as well when she traveled to the present in the comics as well.