I'm not sure what you could swap out for it, but I think Urza's Incubator would be a good idea for this deck. If you choose Eldrazi, it will require a lot less mana ramp to bring them out. If you choose Elf, you can bring most if not all of your elves out for either G or GG. If you choose Elemental, your Soul of the Harvest would be 2GG and Regal Force would be 2GGG.
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And, the 1x Sol Ring restriction is not good for the deck. Casual is playing with no ban list, so I run 4 Sol Rings for extra mana ramp.
Thanks for the idea, but the whole point of this deck was ramping into Beacon of Tomorrows and Planar Portal. I know Sol Ring is restricted, but this is for strictly casual play, and I agree with you, that casual players playing against it would hate it, but I made this deck for a good time when I had nothing left to do.
A new suggestion that will hopefully be more helpful is an Altar Golem. With 31 creatures, this is obviously a deck geared toward getting a lot of creatures, and if you can produce enough mana for Blightsteel Colossus and Emrakul, the Aeons Torn, I think you should add this card.
Mogis, God of Slaughter FTW!!!
What's missing is the fact that, in the main deck, there is no way to effectively play Seismic Assault, if you can either draw it or Treasure Hunt, and Treasure Hunt gets rid of the land unless you're lucky enough to get the Seismic Assault early on (but like I said earlier, you still can't play it.)
I think you should swap one guardian for a Polukranos. Even if you can't get it monstrous early a 5/5 for 4 is pretty good.
If no one has already suggesting it, I would swap out the Searing Spears for Lightning Bolts. Lightning Bolt does the exact same thing as Searing Spear for one mana less. It could also work well with Brimstone Volley. Using Lightning Bolts would probably allow you to use the morbid ability on turn 4 instead of 5.
Have you ever thought of putting in Gaea's Liege? He can serve 3 purposes (one of which works toward another): Turning opponent's lands into forests so that they don't have enough mana of a specified color to play some more expensive cards (say Leviathan from the Beatdown set). That will help him get his power when he attacks. Or with enough Forests you can have him be an impressive blocker. Plus he's a pretty cheap card, only a few dollars at most. I know that he's not an elf or an eldrazi but still, I think it would serve the purpose of the deck.