If we start such a strictly LBT RPG I agree that there ought to be some rules we ought to agree on (only for that particular RPG).
My suggestion for the most basic rule is:
Try to keep the story in a way so it could easily be made a movie without any contents which wouldn't appear in a land before time story.
Among that stuff that would not appear in a land before time story would be humans, any whatsoever kind of fanfiction stuff, any characters from other stories.
I strongly suggest to keep aliens from outer space out of it. One may hold against this point that we probably had two in LBT 7, but I must say it seemed almost like a non-LBT element in an LBT movie.
Also there ought not to be any magical powers or supernatural abilities whatsoever. Rumors about such stuff perhaps (that was definitely in several LBT movies), but nothing too tangible.
As for violence I think it mustn't be banned from an LBT RPG. We have fights in LBT movies, we have deaths... but lets include that for stories' not for gorries' sake.
Are there any further points or points I named which need to be discussed? Who would like to give such an RPG a try?
EDIT: Apparently Roger brought up a set of rules while I was typing. Let's see to what degree we agree :^.^: