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I believe the most basic tiles you would need for any biome would probably be:
The Ground
Some decoration like maybe rocks, grass and flowers?
Trees
Maybe Mountain walls? To work with the cave tiles you got to be able to transfer from caves! :D

Hills, Buildings and other things could probably be added in an update or at a much later date if you intend to upgrade them!
These are just my suggestions!

I love your Fantasyville tiles! They're amazing! I'd love to see some different biomes as well! Such as snow, desert and perhaps withered/haunted trees!
Keep up the good work!