If they do reappear, it likely will be in the Crystal Empire. It's away from the Mane Six, and has a love-powered crystal in its center. While cautious of Prince Shining and Princess Cadence, who both have proven capable of solely defeating them, they also would be in a position to feed off the heart everyday (disguised as citizens, they simply would come off as enthusiasts rather than suspicious figures). Since the Crystal heart has much more energy to give than any single pony, and constantly powered by the crystal ponies everyday, the changelings would be in a position to grow astonishingly in power over the long run....



Hmm... changelings defending the Crystal Empire not because they care about the citizens but because they care about them powering the crystal heart, and by extension feeding them....



That actually would be interesting, and quite grey territory if done (particularly if most of the royal guards in the Crystal Empire happened to be changelings).