How about something new instead of rehashing everything old and cashing in on the nostalgia? Seriously, is it that hard for WotC to come up with some NEW ideas that they can rehash ten years from now?
Now that the rant is out of the way, there's only one Expedition book that I would be interested in, and that would be Saltmarsh. They've already laid the groundwork by fleshing out the town in great detail in DMG2. Now it's time to follow through, or at least let someone else fill in the blanks.
My second vote would go for Desert of Desolation, but I have serious doubts that a revisit to that particular series would be executed as masterfully as it was originally. Some things are just better left alone.