Oculus

Another way to go is to splash white for Safe Passage to make Destructive Force a one-sided wiper; you also get the Kor Firewalkers and vaious ways of dealing with threats (Journey to Nowhere/Condemn/Revoke Existance).
Oh, and this really isn't the best deck for Koth to running around in.

Outside of standard (which I hate by the way), I would consider playing Mark of Asylum, Wildfire, and perhaps Realm Razer.

You'll need some badass mana-fixing with only three lands of each color; you could almost risk playing four Celestial Dawn along with some generic fixers like Mana Cylix and Prophetic Prism, then playing mono-white.

If you can get the Skyfishers out consistently, then Delaying Shield can be very powerful. Keep in mind you can bounce a creature with Evoke (Wispmare and Reveillark, for example) before you sacrifice it with Whitemane Lions.

Maybe you can fins space to cram in the Insurrection/Breath of Fury combo? Insurrection might even be worth playing on its own, or to fill those last sideboard slots as a win condition...

True - I might keep the Paradise Mantle in, they really speed up the Gilder Bairn, and I can always use Shape Anew to get either a Contagion artifact (which acts as removal) or the Reactor. Sleep is probably a good idea, and maybe also Guard Gomazoa.

Depends how often you draw the Excavators - Archive Traps and Tome Scour should become quite irrelevant anyway once you get the ball rolling.

Rite of Replication? It's a Clone at worst, at best the Excavator kills virtually everybody on the same turn. You should probably also play Journey to Nowhere or another piece of removal, just in case.

Blademasters are still decent cards by themselves...

You know, the easiest way to have both Ogres and Demons out early-game is with a Firebelly/Skeletal Changeling...

Maybe sideboard in Autumn's Veil (if you use sideboards), they can be helpful in breaking tough control decks. Khalni Heart Expedition should also come in handy.

Maybe - Surgespanner does a nice job of keeping slow opponents in check, but does eat up mana...

Not exactly standard for the Seat of the Synod, but that's easily fixable. Etherium Sculptor is probably worth playing, as is playing black for some tutoring.

Interesting idea - maybe dredge up a few more cards in a similar vein to Devastating Summons, or something to color-fix your rituals (like Manamorphosis).

I know somebody with a deck almost identical to this one, except he plays Barbed Shockers and Terminates.

Wither and Infect don't work, as they are a form of damage; however, creatures with protection can be dealt damage if the damage can't be prevented (eg. Unstable Footing).

Omnath, Locus of Mana is another form of upwelling, as well as a win condition.

It does work - protection prevents damage, can't be targeted/equipped/enchanted, and can't be blocked. It only prevents damage, though, so Damnation and Infest will still kill creatures with protection from black.

I probably don't need any more lands; I end up drawing more than I can play already. I'll try make some space for the Treespeakers though.

You can probably dump white. Play Scion of Oona to protect from removal, and Mistbind Clique. If you have the budget, Sower of Temptation is also worth playing.

Look up my Maralen's Orb deck. Basically, it uses Guard Gomazoas to hold off, Magus of the Future and Crystal Shards to keep playing, Memory Lapse as a solid counterspell, and Wheel and Deal to destroy opponents' hands.
I play Lightning Greaves to protect Maralen, but Shielding Plax is also good. Cards like Augury Adept means you can still "draw" cards, and Blood Tithes effectively steal a turn from each opponent. Phylactery Lich is probably worth considering, depending on your build.