Going off of Mojko's suggestion, what if the keyword effect draws a Rare Nature not if there isn't another Nature in hand, but if there is not already a Rare (Nature or perhaps any at all) in hand?
Let me rephrase, since that sentence ended up being a bit of a mess. If a Nature is played after a non-Common Nature and there is not already a Rare in hand, the next card will be a Rare nature.
Last game I played against a Nature deck, my opponent was drawing several Rares by use of Uncommon Nature cards costing only gems followed by
Lake Fairy to discard keywordless cards. He easily filled his hand with Natures while building recruits and finished me with
Volcano and
Forest Dragon. All I could do was build until his resources caught up to me.
Edit: I see Lord Ornlu had a similar idea and got it in while I was posting. ;)