I forgot about Pterano. Another underrated character that doesn't get enough screen time.
The only thing about Pterano coming back is that he would be violating the terms of his banishment. The valley wouldn't take kindly to that, especially Mr. Threehorn. Having said that, I agree that Pterano should make another appearance in some shape or form.
I don't agree with replacing Chomper. He has made the most appearances out of all the secondary characters, being in two movies as well as the TV series. I would actually take out Ruby and replace her with Shorty. Despite only being in one movie, Shorty was actually a pretty in depth character. We got to see how jealous he was of Littlefoot for spending time with Bron, and in a way probably threatened in the sense that he would lose Bron to him. I was very happy that they were able to make amends, because now Littlefoot would finally have the brother he was looking for. The episode that Shorty was featured in did bring that sense of jealousy back when Shorty didn't like how Littlefoot was going through the Big Longneck Test and not him. I would have liked to just see him more.
I'm a little stuck on whether or not I would want Ali to be brought back. I know some people think she was not out of character when she returned in the TV series, and I completely see the arguments to back that up, but I just really didn't like how she pretty much shut out their adventure together and idolized Rhett. Again, I can understand why she did, but at the same time I didn't like it at all. The episode didn't even really feature her that much, and yet she was supposed to be the focus. So if she were to come back, I would have at least taken the events from LBT IV into account, and also given her much more screen time.