“I don’t know if anyone has noticed this, but often the hints to an episode’s ending are given in the earlier parts of the episode...
That’s…less easter eggs and more just plain good writing? In a closed story arc (particularly for short stories, and you can consider each episode of a show like Gravity Falls to be its own short story), the ending is generally supposed to be reasonable, but at least somewhat unexpected. Or, it can be predicted (and hinted at), and you might guess the ending correctly, but it’s not a certainty (and there’s always exceptions to any rule, and not being structured like this doesn’t necessarily mean something is bad, but I digress). So all those instances are foreshadowing, really, and it’s a testament to how good the writing in Gravity Falls is.