So the first dungeon in the game is a sort of Mt. Moon scenario where you can pick up fossils, however due to the largeness of the region I think it would be a good idea to have multiple fossils similar to RGBY so for this we're mainly looking at coming up with an idea for two fossils that wouldn't necessarily be super strong Pokemon but would add variety to the player's team.
The cavern is a coastal one so fossils based on Sea Life is fine.