I have seven plots in mind six of which I think could be used in movie (with the seventh one being a sequel to a story that contradicts LBT 10). More than one of the plots I'm thinking about contain gloomy elements, but certainly not coming as some kind of massacres or the like which I think don't have a place in the land before time.
As for characters I could imagine for example a scavenger to show up. This sure would cause quite some difficult topics to deal with. But we had herbivores, carnivores, eggeaters (omnivores), and I don't see why a scavenger should be kept out. Without scavengers ecological systems wouldn't work.
I could imagine scavengers to be sort of a fringe group, scorned but tolerated. On this basis I imagine them to be quite thick-skinned, withdrawn, and cynical. Still they are not necessarily villains.
Another character I could think of would be a feathered bird. With all its advantages (less vulnerable) it may be that Pterosaurs envy the bird. Conscious of his advantages such a bird might be vain, somewhat arrogant, and perhaps a little stupid.