The ending leaves you wanting more, but there is no signature tag at the end. I admit, it's disappointing, but the movie is so self-contained, that it doesn't really need one — but if it did have one, what would it have?
If I had it my way, I would have the post-credits scene focus on Spot's family. During the reunion, audiences will barely get a glimpse of them. All that is shown is that he has a mother, father, brother and sister. They hug, reunite and go off into the wilderness — and that's it. It would be nice to get to know them a little more. And honestly, Spot was my favorite character. I totally wouldn't mind a spin-off movie with Spot and his family — and a post credits scene would have been a perfect opportunity to set that up.
Arlo's story is resolved and it's absolutely heartwarming to see him reunited with his family — but has he really been cured of all his fears? In the beginning of the movie he is scared of the chickens (or mutated versions of) that he has to feed in their coop — and if you see them you'd be scared too. Has he totally overcome his chicken phobia? A funny little stinger at the end showing him interacting with the chickens would have been a treat.