How can I improve this deck? Cards I am considering adding but haven't because that makes it less probable that I'll draw what I really need are: All is Dust From Beyond (notably, it makes scions, not spawns) Belbe's Portal Plummet Naturalize
Have you considered Might of the Masses or Strength in Numbers?
These would go well with all the tokens and they would add a surprise factor to your deck.
1) Spawn lots of tokens
2) Play big Eldrazis
3) Kill your opponent
...I would try to skip either step #1 or step #2
Right now, your opponent can disrupt step #1 with minor damage spells like Pyroclasm or Arc Lightning and distrupt step #2 with targeted removal like Swords to Plowshares.
Also, the process before you end up killing your opponent is fairly long.
If you skip step 1 and go with a 12-post deck (Cloudpost and friends), an urza tron deck (Urza's Tower and friends) or you use some artifacts (Thran Dynamo, Basalt Monolith and similar) to have the mana to spawn the big Eldrazis, the ressources you need to spawn Eldrazis will be harder to deal with and wont die when the Eldrazis get spawned.
Otherwise, if you really want to stick to your current deck strategy, here are a couple of suggestions:
Steely Resolve: Make your big Eldrazis shroud...that way you won't have to worry about single target removal (Infernal Reckoning and friends) when you manage to play your Eldrazi. Druids' Repository: I assume this deck is made to play multiplayer games. In multiplayer games, there is always someone who has nearly no defense...you can attack that person with your tokens and bank up mana that you will later use to play your Eldrazis.
Choose Eldrazis that give a good benefit even if they die...It that Betrays, Bane of Bala Ged and Ulamog's Crusher look nice but if they die to single target removal, you have nothing to show for your efforts. The bigger Eldrazis (Kozilek, Butcher of Truth, Ulamog, the Ceaseless Hunger and Kozilek, the Great Distortion) will give you a nice benefit even if they get countered or killed before you get to attack with them.
All is Dust
From Beyond (notably, it makes scions, not spawns)
Belbe's Portal
Plummet
Naturalize
Here's the deck:
4 Nest Invader
2 Broodwarden
4 Artisan of Kozilek
4 Hand of Emrakul
1 Desolation Twin
1 Ulamog's Crusher
1 Bane of Bala Ged
1 It That Betrays
2 Kozilek's Predator
1 Awakening Zone
3 Canopy Cover
Sorcery
4 Brood Birthing
3 Growth Spasm
Instant
2 Spawning Breath
1 Return to the Earth
4 Deathrender
1 Spidersilk Net
Land
1 Rogue's Passage
10 Forest
10 Mountain
These would go well with all the tokens and they would add a surprise factor to your deck.
Also 4 Deathrenders might be a couple too many?
And some 1 cmc cards could help out - Elvish Mystic, Wild Growth/Utopia Sprawl?
1) Spawn lots of tokens
2) Play big Eldrazis
3) Kill your opponent
...I would try to skip either step #1 or step #2
Right now, your opponent can disrupt step #1 with minor damage spells like Pyroclasm or Arc Lightning and distrupt step #2 with targeted removal like Swords to Plowshares.
Also, the process before you end up killing your opponent is fairly long.
If you skip step 1 and go with a 12-post deck (Cloudpost and friends), an urza tron deck (Urza's Tower and friends) or you use some artifacts (Thran Dynamo, Basalt Monolith and similar) to have the mana to spawn the big Eldrazis, the ressources you need to spawn Eldrazis will be harder to deal with and wont die when the Eldrazis get spawned.
If you skip step #2, there are quite a bit of cards that can make your tokens a real pain for your opponents (Beastmaster Ascension, Eldrazi Monument, Glare of Subdual, Aura Shards,Overrun, Windbrisk Heights, etc...) and you can end up killing your opponent with these tokens.
Otherwise, if you really want to stick to your current deck strategy, here are a couple of suggestions:
Steely Resolve: Make your big Eldrazis shroud...that way you won't have to worry about single target removal (Infernal Reckoning and friends) when you manage to play your Eldrazi.
Druids' Repository: I assume this deck is made to play multiplayer games. In multiplayer games, there is always someone who has nearly no defense...you can attack that person with your tokens and bank up mana that you will later use to play your Eldrazis.
Choose Eldrazis that give a good benefit even if they die...It that Betrays, Bane of Bala Ged and Ulamog's Crusher look nice but if they die to single target removal, you have nothing to show for your efforts. The bigger Eldrazis (Kozilek, Butcher of Truth, Ulamog, the Ceaseless Hunger and Kozilek, the Great Distortion) will give you a nice benefit even if they get countered or killed before you get to attack with them.