This is the Druid mid-range deck that I used to get from rank 4 to legendary.

Check out my stream at http://twitch.tv/faction60 to see this deck in action

It's focused on early/mid game board control into a savage roar finish using the tokens generated by Violet Teacher and Imp Master, but can also hold its own in long games due to 2x Anicents of Lore and Cenarius.

Legendary replacements:

Replace Leeroy Jenkins with a mid-game drop of your choice. An Auctioneer or Druid of the Claw for example.

Nat Pagle is not really easy to replace, I'd say he wins 1 out of every 10 games by himself which is huge for a card that's 1 slot in a deck of 30. Replace with Thalnos if you have him, otherwise I'm not too sure.

Cenarius should probably be replaced with an Auctioneer

If you don't have 2 of these legendaries, I would definitely suggest going with 2x Argent Squire as they are very good against hunter and warrior aggro and can survive for a long time to be buffed by power of the wild or savage roar.

Bad matchups: Reynad Aggro Warrior, Shaman with good lightning storm RNG, Giants Warlock. All of these are winnable depending on draw luck, however.

Good matchups: Aggro warlock, mid-range warlock, Standard Rogue, Control druid, Aggro hunter

Mulligan strategies:

Mulligan hard for 2 and 3 drops against almost every class. Against aggro warlock keep a swipe and any claws/wrath/2 or 3 drop minions, and against aggro in general keep the claw/wrath. Always keep innervate no matter what the matchup is, and if your hand has innervate completely change your mulligan strategy, ie: keep 4 and 5 drops that you plan to innervate.