Have you considered Aven Mindcensor and moving Spirit of the Labyrinth to the main over four copies of Mirran Crusader ? Leonin Arbiter would be another choice.

With library search denial, you could play Ghost Quarter s. Those also synergize with Flagstones of Trokair since you can hit your own lands and dig up two Plains .

I like Serra Avenger as your other choice because you can cheat it out with AEther Vial . I don't know that I'd play a full four, though.

The other cards to consider are Brimaz, King of Oreskos and Mangara of Corondor . You're also missing a 60th. card. I don't know if that was intentional until you could find something to fill that slot, but I'd recommend that be a 23rd. land in the form of Horizon Canopy . The draw power is relevant if you get flooded.

Drop in 4 Horizon Canopy and 4 Rishadan Port where the Mishra's Factory and Flagstones of Trokair are when you can.

Think about Cataclysm in the sideboard. It helps your matchup against control/midrange decks. Manriki-Gusari is great against other Stoneforge Mystic decks.