That certainly is an intriguing thought. I had wondered before, as well, exactly how much time it took Littlefoot and his friends to get to the Great Valley - and not to mention what other obstacles they must have faced during their journey.
I've always sort of looked at it from a "cut-and-paste" perspective, I guess you could say. That anything and everything we didn't see in the film is, maybe purposefully, left to our imagination and that it's up to the viewers to fill in the blanks. In my opinion, it really could be taken many different ways...
For instance, as far as behind-the-scenes scenarios that might have happened to Littlefoot and his friends: how would Don Bluth have created the additional parts of the story? How would fans/viewers have created it in their minds? Or how would todays' filmmakers tell that untold part of the story?
There's so much to consider, which makes it such a compelling topic.
I'm going to have to share the others' opinions though, in that it really would be a "double-edged" sword to actually try and create a prequel/midquel. I doubt it would ever be done, but it still would be interesting if was carried out. Technically, it really could be done - not just done, but done well - it's just sadly unlikely because of the way that people tend to make prequels/sequels. I imagine, that if a Land Before Time movie was made that told about the other dangers they faced while traveling to the Great Valley, it might end up being a watered-down version of the other sequels and wouldn't have the same feeling that the first movie had.
If anything, one interesting way to get a look into the unseen dangers - at least, a version of them - would be if someone would write a good, believable story about Littlefoot and his friends' adventures in the Mysterious Beyond.
In fact maybe there already exists such a story; I don't know. (It's been a while since I last browsed the LBT section of FanFiction.net, which seems to be where most LBT fanfiction is found.)