You could O-ring/Fiend Hunter an Inferno Titan, Vexing devil, Chandra, Hellrider, Reckless Brute, and I'm sure many many more in standard. If you wanted to get technical or keep going after you use Worldfire, you could exile an Elderscale Wurm with the O-ring/Fiend Hunter. Just some ideas.
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Kokusho is banned in EDH.
This deck is illegal. You can't have a card in your EDH deck if it has a colored symbol anywhere on it that isn't in your Commander's color identity, nor can you add mana of a color not in your Commander's color identity, thus making cards like Wall of Tanglecord illegal. Also, Memnite can't be your commander. He is not a legend. Unless you mean Memnarch?
I had a Rainbow deck for a while. I'd put in a Manalith or 2. They're alot better than the Obelisks, and the CMC is the same. Obelisk of Alara is another good one. And yeah you should get some weaker creatures in there for a much better early game. I'd also recommend more instants and sorcerys like Terminate, or maybe even a board wipe. I don't know, just some suggestions.
Pretty nice. I'd look for a couple Woolly Thoctars and an Obelisk of Naya. They'd fit in nicely.