I like the title.
I guess Spike visits like Ali, Bron and Shorty do in the tv series. How about Chomper. He could, perhaps, live in the valley eating what he did before, and maybe fish. Or he could visit the valley at times if he lives outside it. If he visits he, perhaps, would have some sort of signal he gives so the folks who patrol, if any do, would know it is Chomper, though some would still gather to greet him and make sure it is him. Also has Bron found a mate? Does Littlefoot have younger siblings? Cera have more then just one?
As for Littlefoot that is a good idea. It could be the time when eventually he'll have to decide to stay, or join Ali or Bron's herd, and Ali may have to make a decision as well, based on what Littlefoot does, they'd have to talk about it, though that will likely be after the rp takes place so no mention may happen.
I guess with both longneck herds in the valley for now that would add in Shorty & Rhett into the playable characters.
I guess Tricia is roughly about Cera's age in the movies and series. Littlefoot & Ali may be a bit to young to have had hatchings maybe, up to you though. Same with Ruby, if she found a mate.
Not sure if you'd want to discuss some possible fc char personality changes or leave that completely up to the player, or have the player discuss it with you either here or by pm.
Maybe a list of playable characters, something like, though you will change it being gm this is just an idea to get you started based on your info so far:
Teens: Littlefoot, Ali [to young to have had hatchlings yet?], Shorty, Rhett, Chomper, Ducky, Cera, Petrie, Guido (? older teen ?)
Possible Older teens: Ruby, Guido
Kids: Tricia (any other kids Threehorn and Tria may have had)
Oc's:
kids: the 3 kids of Cyrix and Mim,
teens: Denton, Longtail
Slightly older adults (roughly 30's?): Mim & Cyrix (likely still as physically capable as in the character section I'd guess)
Similar to my idea, though I was thinking of doing the gang as adults with their own kids about their age in the movies. So a similar idea.