Unfortunately, we have no idea when the expansion will drop. If you're the optimistic type, yesterday's BlizzCon announcement might have hinted to you that the expansion will come out before the big event in Anaheim, which means we'll be in Northrend before October. I'm not so optimistic -- I'm still thinking we won't see Wrath until right before the holidays, and Blizzard being Blizzard, probably delayed until January again.
But either way, you've got at least a few months, so what are your major goals before the expansion hits? Going to level a character, get a certain Arena rating, or do every Heroic? Aiming to get your mount, still, or have a goal with your guild to finish Sunwell before all the new content shows up? What are you aiming to do before going to see Arthas?