This has been a matter of contention of mine lately. Each time a new series/season is announced, the first thing a lot of people go after is wondering what story threads, locations, or characters that could be seen/visited/mentioned from an earlier show.
Now I will obviously confess that I've done this a lot myself, but, I also want the shows to carve out their own paths and form their own mythos. Sure, if it makes sense to their stories to have something from an earlier show appear, fine. But I'm really starting to get bothered with the ever-growing desire from the audience to go back to what came before and give the audience an update simply because they simply could.