All signs point toward the Sleeping Forest once leading to the Ancient Forest, which in turn then led you out to Coral Valley. I'd rather implement this than create completely new caves.
But at present I'm more interested in finding the coordinates to properly teleport on to the walkmesh where the unused cave exit/entrance is. However, all my attemps to teleport to the right spot have failed.
The Ancient Forest theory is supported by:
1) Areas and their field identification numbers being generally ordered in the same order that you visit them throughout the game and the ancient forest being placed after the sleeping forest and before coral valley.
2) Unused "Ancient Forest" tutorial text in Bone Village.
Example:
“If you learn the special qualities
of the animals in the (Old Forest),
there'll be nothing to fear.
Do you want to hear more?”
-> Yeah, tell me
-> Not now
“What do you want to know about?”
-> About Pitcher Plants
-> About Scent Balls
-> About
-> About Inchworms
3) The monster attack IDs, which also line up with the Ancient Forest theory.