For some reason, I always think of Hedron Blade instead of Ghostfire Blade, but I fixed that. I also took out Thran Turbine and Scarecrone, in order to put in Vedalken Orrery and Unwinding Clock. Let me know if there're any other tweaks, I largely made this just to get ideas onto paper.
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Great ramp deck! The only things I might add would be a Nykthos to take advantage of the power of mono green devotion, and a Helix Pinnacle for shenanigans with Omnath, and an early game win condition.