doctorhydrogen

It's a fantastic card unto itself, but Sylvan Library comboes with Abundance to let you keep all three cards.

Thrumming Stone would be strong here

Detention Sphere is probably going to be better than Oblivion Ring for that type of effect just because it does everything O-Ring does but can also on occasion hit a swarm of tokens, or take out an Avacyn and the copy someone made of Avacyn, etc.

It's probably also not going to matter much, but Void Shatter has the same cmc as Cancel plus it exiles the spell, plus it can't be hit by the occasional Red Elemental Blast you see in a mono-red deck. If Void Shatter was a three dollar card I'd say who cares for the 1 in 100 games where it matters, but since it's like a twenty cent card there's really no reason not to switch.

Myriad Landscape and Wayfarer's Bauble are one of the few ways you can land ramp in W/U. I'd consider both. Also since you have plenty of draw I'd consider finding a slot for Terrain Generator. I've never been unhappy to see it in my W/U sphinx deck.

I've been having great luck with Overwhelming Intellect lately. The trick is to not think of it as a counterspell; it's a terrible counterspell to need to use to save yourself. The trick is to think of it as a draw spell that also sets your opponent back a card and mana. If you're holding six mana open for it hoping to save yourself from something you'll always get burned, but if you hold six mana open because you looked around the table and saw that at least one of the other people was gonna try to cast a creature next go around (which is almost every single round) then you can just think of it like a Blue Sun Zenith that ruins their turn and fills your hand.

Thought Vessel seems like overkill. I probably draw more cards in my Sigarda enchantress deck (I have Enchantress's Presence which you don't, and I run stuff like Flickering Ward for repeated draw and Hunter's Prowess beyond what you have) than you will here, and there aren't many times I can think of where Reliquary Tower wasn't enough. It's not a bad card, and I'd run it if EDH ran 125, but I have a hard time including it in a 100 card format.

Obviously Serra Sanctum is great, but it's now a 50 collar card which is annoying.

Replenish is better than Retether in almost all situations. Yeah, it's 13 bucks, but it wins games. Then again you're only running a couple of non-auras, so it's probably not THAT much better in this particular deck, but it's still better.

Sigil of the Empty Throne can get out of hand in a deck like this.

Short of an aggro deck like Edric or something I'm not a fan of mana dorks like Birds of Paradise or Bloom Tender. I guess it depends on the meta, but I hate seeing my ramp go away to the inevitable turn six board wipe. I'd much rather run a couple of mana rocks or land ramp in those slots, or even enchantment ramp like Wild Growth and Overgrowth (which feed Serra's Sanctum, Ancenstral Mask, Ethereal Armor, etc.). But like I said, that's meta dependent. I see a ton of board wipes, and not that many mass-enchantment wipes or mass land destruction. Maybe you get Birds of Paradise to stick for 8 turns.

I've found that in a three color deck Exotic Orchard almost always is a dual land, if not an outright tri-land in multiplayer games.

I ran Atarka dragons for awhile in EDH:

Urza's Incubator is a trap. Unless you cast two dragons a turn (which is almost never going to happen) you're better off running a Worn Powerstone or Thran Dynamo. It's easy to check. Next time you cast Urza's Incubator check every turn and see if a Worn Powerstone or Thran Dynamo generates more mana than Incubator saves. I'd wager than in 9 out of 10 situations it does.

Greater Good is imo a must-include. It makes losing a dragon a lot easier when you can draw 6 cards off it.

Bower Passage makes all your guys virtually unblockable.

Mudslide is going to screw over a bunch of people who aren't you.

Earthquake and Magmaquake are going to kill a bunch of guys not on your team.

Gravitational Shift seems like it would be good here.

IMO Kor Haven is worth a slot here. You can afford the colorless mana.

I guess it's probably meta dependent but I personally would run Revoke Existence or Fate Forgotten over Disenchant. I reject the notion that your Theros God or Darksteel Plate or Blightsteel Colossus should be immune to my removal, and I reject the notion that once I remove something you should be able to get it back from your graveyard.

IMO Urza's Incubator is a trap. Given the high cmc of your angels and the fact that even with Avacyn you only have 10 total in your deck you're probably rarely ever going to cast two a turn, which means that absolute best case it saves you 2 mana per turn, and there's going to be a lot of turns it doesn't do anything. Plus it affects ALL creatures of that type, so there are times it will help other people. On the other hand Pearl Medallion is going to affect 44 spells, quite often multiple ones per turn, but at least probably 1 every turn. Or just run a mana rock like Worn Powerstone. Regardless, Urza's Incubator is almost always the worst option.

Nykthos is going to tap for 5+ mana often enough in this deck that I think it's worth running Expedition Map to fetch it.

Not a big deal but since you have no regeneration Wrath of God is technically better than Day of Judgement at the same CMC.

Condemn was amazing when it could tuck a commander, but now that it can't I don't think I'd run it over something like Crib Swap. There's just too many omg-need-to-remove-this-now cards that will never attack for you to Condemn (Consecrated Sphinx, Jin-Gitaxis, Sheoldred, Terestodon about to get blinked a few times, Prophet of Kruphix, the list goes on and on).

I think mono-white needs draw enough to run Mind's Eye.

Not sure what I'd pull for either, but Mass Calcify and Crackdown would generate good value here.

No joke, I opened my web brower to the MTGVault page to see this deck on top of the list as my wife and son play Bop It on the couch next to me. That's eerie.

A friend of mine runs a Dromoka deck against me weekly:

Hunter's Insight and Soul's Majesty are stupidly good draw spells here. You already have Hunter's Prowess. They're risky cards in other situations but here Soul's Majesty is going to almost always be one mana more than Harmonize for a least two more cards.

Do you have ways to re-use/abuse Spine of Ish-Shah I'm not seeing? As far as I can tell it's a seven mana sorcery-speed removal spell in colors that have great removal. At the very least Scour from Existence is instant speed AND an exile for the same mana. Personally I'd just run Beast Within.

41 land is more than enough for a two-color deck. You could run 38 easily.

I'd run Dissipate over Cancel. Not a huge deal, but I hate seeing something I counter get recurred.

Fact or Fiction > Steam Augury. Nothing wrong with SA but I wouldn't run it over FoF.

Chaos Warp is too useful to not run in a deck that has to react. It can clean up anything you miss.

Kokusho was unbanned in September of 2014: http://forum.mtgcommander.net/EDH_Forum/viewtopic.php?f=1&t=17210

Vigor is stupid good in a +1/+1 counter deck.

I'd rather pay 1 more for a Bident of Thassa and know it drew off every creature than have Bred for the Hunt not work on Vorel himself, Prophet of Kruphix, etc.

I do think Hardened Scales is a must-include. So much value for one mana in this deck. Personally I'd run it over Primal Vigor because that card burned me a lot by helping other people, but that probably depends on your meta.

I'd run Deglamer or Unravel the Aether over Naturalize. I want to be able to hit Theros gods and Darksteel whatever and a Blighsteel Colossus or stuff protected by Avacyn, and Naturalize doesn't let you do that. Plus Deglamer and Unravel don't cause enter-the-graveyard triggers, and stuff is harder to get back from a tuck than it is from the graveyard. You're almost never going to have a Deglamer in hand and wish it was a Naturalize but there are a gazillion situations where you could be holding a Naturalize and wish it was a Deglamer.

Exsanguinate is almost a win-condition unto itself, and it's on-theme in vampires.

I ran Vorosh +1/+1 counters for awhile before converting it to Mimeoplasm. And by converting I mean changing commanders :)

You should run Ob Nixilis, Unshackled. Not only is he a decent +1/+1 dude in his own right, but forcing someone to tutor up lands with Fertilid while he is in play will absolutely be the most hilarious win you get with this deck.

It's pretty easy to hit Threshhold off Mosswort Bridge.

I know it's not as strong as Doubling Season, but Hardened Scales does amazing work.

Draining Whelk is a nice way to sneak a counterspell in with a +1/+1 counter guy.

Cauldron of Souls is stupid in this deck. Since +1/+1 and -1/-1 counters cancel each other out, most of the time you use Cauldron you can just use it again next turn if you need it.

Gilder Bairn and Vorel of the Hull Clade are nice ways to double up counters.

My version of the deck if you're curious: http://www.mtgvault.com/doctorhydrogen/decks/the-mimeoplasm-edh-9/

There's also Wing Splicer, Master of the Pearl Trident, Merrow Reejerey, Merfolk Sovereign, and Myr Matrix.

I built a variant on this on paper once but never actually did it. Mine was utilizing almost all lords (Lord of Atlantis, Master of Waves, etc).

Swarmyard would be a nice way to regenerate your commander. Elephant Graveyard would work too if you don't mind spending $25 bucks.

Dissolve is strictly better than Cancel.

For one more mana than Rush of Knowlege you'll probably draw 2x as many cards with Recurring Insight.

Druid's Repository would generate stupid mana here, plus it makes it a ton easier keep counterspell mana free.

Edric's main enemy is board wipes. Teferi's Veil lets you give zero fucks about them, particularly uncounterable stuff like Supreme Verdict.

Natural Order is a bomb in a lot of decks, but here is there anything game-changing you're going to fetch up besides Craterhoof? And if you already have that card in hand NO isn't going to do much. Me, I'd just run a win-con unto itself in that slot like Biomass Mutation.

Dudes to consider Blighted Agent, Dryad Sophisticate, Hypnotic Siren, Looter il-Kor, and Thalakos Sentry. A couple of those guys are rogues to help with Notorious Throng, too.

Fastbond is banned in EDH.

Hall of Gemstone in mono-green won't hurt you a bit, and it will totally shut off some decks.

Primal Order would do good work here.

Druid's Respository is stupid in Prossh. 7 (or 9, or 11, or whatever) free mana before you sacc the kobolds? Yes please.

With that many dudes Lurking Predators would be stupidly good.

Find a slot for Sylvan Safekeeper. It's usually my first grab and makes it really yard to deal with Yisan.

I usually grab Soul of New Phyrexia at 6. Since you have Seedborn Muse probably at 5 that pretty much ends the game right there since short of a mass bounce your guys are shrouded from Sylvan Safekeeper and indestructible from the Soul.

Craterhoof Behemoth in the 8 mana slot means "kill five people simultaneously".

Panglacial Wurm is fun as you can cast him mid-search.

In this deck, Primal Surge would be an alt win condition as it would mostly read: put 30-40 permanents directly into play. I ran Akroma's Memorial and Concordant Crossroads just give them all haste after.

Quest for Renewal is a nice way to get bonus Yisan untaps.

Hall of Gemstone just turns off some decks, and doesn't hurt you a bit.