There are a ton of F2P decks that are mythic ready. Copperpitch just did a video on a magic deck that he played on stream in weekend rank at mythic and was winning. There are a plethora of nature decks out there. You just need to decide if you want a more aggro type nature deck or the big beast route. Just be mindful with armor in the game a lot of zoo decks have trouble dealing with that. So you need a spell or something that can handle a creature that might end up having a stack of armor on it.