I foresee post-threesome fights, some Jenny vs. Eric (like Eric saying this new guy is not good for her) and Serena and Tripp deciding that it's not a big deal if they make up for a while cause he's going to Washington and therefore it will be over. Oh and I foresee little Blair and less Chuck. I also expect there would me some minor plot to keep Lily and Rufus alive.
I really hope it won't be a Olivia-Dan-Vanessa threesome, though I'm afraid it's where their plotline is stirring them. Btw what happened with people in Canada who used to see the episodes before the rest? Is it still the case? No spoilers?