Looks very cool. I personally run Lightning Maulers in my RW casual humans deck, but I don't know if that would mess with your match up. I'd also think about adding one more Zealous Conscripts, as their ability to gank a planeswalker before it ultimates is phenomenal.
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I'm not really keen on Harmonic Sliver, mainly because it's mandatory. It's the same reason I never run Manic Vandal. True if I drop the Metallic Slivers, I wouldn't have to worry about any of my slivers getting destroyed, but I also lose four one drops. As far as lands, I was pondering throwing a set of Ancient Ziggurats, but I'm not sure if I should drop some of the land or what...
I love being able to flash out my slivers, especially a Muscle or Might sliver to buff them up when they're blocking.
I really would prefer not to run Coat of Arms. The slivers have enough buffs for my tastes, especially between Sliver Legion, four Muscle Slivers and Might Sliver. Plus, most of my friends run Elf, Goblin and Merfolk tribal decks, in which case Coat of Arms would work against me. The Changeling combo is a good one, I might look into it. I hadn't previously heard of Door of Destinies, I might throw that in there.
I really like this deck, but I think I'm going to cut out a Swamp and Gut Shot for a couple Drogskol Reavers when I build it, see how that runs.