What does this deck do that Valakut ramp cant do by itself? What does this deck do that Eldrazi green doesnt do by itself? My best suggestion would be to stick to a normal Valakut list and run the Eldrazi stategy out of the sideboard. Using the Eldrazi as a plan B, since everyone runs the hate for Valakut you can just smash them games 2-3, when you drop Emrakuls that they didnt know were there.
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cemetery reaper is not a standard card.
Forgot to ad: Also in blacks favor, a few Vampire Hexmages in the sideboard or even main deck is a good way to deal with walkers. Also Ratchet Bomb if you know whats coming just let the counters sit there and watch them squirm.
Black has the best removal in standard right now. Its not just Doom Blade, there is also Disfigure, Grasp of Darkness, Consuming Vapors, Consume the Meek, and Gatekeeper of Malakir. As someone who runs U/B control myself Id say the color you need to question here is White. Aside from Gideon what does it give you that black doesnt? I would also recommend Frost Titan, he takes control of the game if not answered quickly. Other then that your deck looks great. Also I think the tar pit man-land is better but that is opinion.
I just posted a deck with a similar idea, except I used Liquimetal Coating to protect the angel. Whispersilk Cloak is also a very good idea. I may have to play around see whats better.
I like your idea for Guard Duty. Deck looks like it would be a lot of fun to play.