I'm afraid the same thing they always do... We won't have anything explained, and the egg won't hatch. Sorry for being a pessimist, but CLAMP is all about projects which are unfinished, or nothing is revealed in the end. :<
But, well, this is my idea what would happen:
Yuuko will tell Watanuki that waiting for her is pointless, but he won't be willing to listen to her. And the one saving him from a dream would be Doumeki (maybe/hopefully using the egg?).
I don't know, I want the end to be as cool as possible.