"Summon uncommon Nature card"
This is perfectly fine, and in the case of triggering the Nature keyword, you would instead get (arguably better) a Rare Nature. But I foresee people reporting bugs when they see that happening. (My opponent should have drawn Uncommon but got Rare, it's broken!)
To avoid this, perhaps you could select a card in hand to replace with an Uncommon Nature, or have it replace a random non-Nature card in hand with an Uncommon Nature. I don't know if this would make the card too powerful, though.