You should add in the Tyrannosaurus and Velociraptor as well as the Diplodocus/Apatosaurus, Heterodontosaurus and the Acrotholus. And in the long run all the dinosaurs that roamed the earth!!! Here is a list...
Um will t rex be tameable and OOO i know! Parasaurolophus and Spinosaurus and Triceratops maybe add Quetzalcoatus and Pteranodon
PaleoCraft is supposed to make a scientifically accurate model of the dinosaurs, and back 65 million years ago there where no humans to ride the dinosaurs so I would imagine that they would not add dino riding... unless it was going to be scientifically accurate and be more complex than just feeding them beef or other foods (see Pavlovian response)
Prehistoric biomes?? desert with oviraptor and protoceratops, jungles with Microraptor, raptors and small herbivores ( dryosaurus, homosephele), oceans with ichthyosaurs and plesiosaurs, beaches/lakes with parasaurolophus and deinosuchus (bus sized crocodile) and plains biome with t-rex, triceratops and sauropods. I really love this mod, please consiter my suggestion.