Is there any significance to the stuff in the cave? It makes it sound mysterious that someone may have been in the cave.

There was someone, or something, definitely in the cave during the game. If you look at the cave while it's still sealed, every so often you see a pair of beady little eyes peering out from inside the cave. So, someone was certainly in there.

The significance? Perhaps a cliffhanger until VV 2 comes out? :]

Once it's opened and you send in a villager, you'll get a message that it's as if someone has been hiding out there ...

The eyes are just a teaser. As you suspected to let you know that there will be more to come, vv2.