Personally, I like
Halimar Excavator over the
Shapeshifter. It gives you an entirely different win condition without really changing the function of the deck.
Considering how low your overall CMC is for the deck, you could axe green entirely. You could replace Harahan druid with either
Ondu Cleric,
Makindi Patrol or
Kor Bladewhirl for different effects. You could replace the enchantment with another one;
retreat to Emeria. Dropping green could also free up the mana base to run
Evolving Wilds, which you can use on your opponent's turn with Retreat to trigger your allies.
There's a lot of things you could change
Swords to Plowshares with.
Path to Exile does a similar function, but they're pricey.
Just a few suggestions.