Some of the characters are not hard to imagine what they'd be like as adults. For Petrie look at his uncle Pterano, he'd look very similar to him as an adult. Same with Ducky & her mother, Spike and the Spiketail Leader from the 8th movie. Chomper would look similar to his dad in most cases, Littlefoot to his grandfather or father. As for Cera, likely similar to an orange version of Tria, I'd guess.