You could add a Curiosity and Ophidian Eye to the deck as if you enchant Niv-Mizzet you basically automatically win the game and fishing them out isn't too hard as you are drawing a ton of cards.
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You can have more then one legend and multiple copies of them in your deck just not on the field at the same time. The idea is birthing pod will sacrifice the legends to get the better Eldrazi's so you don't ever end up with two copies of the same legendary in play at once. The reason Ulamog and Kozilek each are in there twice is in case you happen to draw one, as birthing pod is unable to fish them out of your hands that would be detrimental so this way drawing 1 would mean you still have another in your deck to cycle around with.
I don't think sigil of sleep would really help as in the test games I have played your opponent really can't do anything they just sit there and watch their life total slowly drain away turn by turn but its really limited in only having viridian longbow for ping. Though once the set up is complete its hilarious to watch your opponent rage about never being allowed to have a hand. The kappa shell could probably be taken out in favor of something else though and splashing blue for card excel that doesn't take life could be a good idea.
I changed up the deck a little cause I saw that Demonic Consultation was banned in almost every format and realized that the Demonic Consultation + Unearth combo wouldn't work so I threw in a Ichor Slick cause I had one laying around and it had cycle and now I hit the sweet spot of cycle cards where I almost always win on turn 3 because I don't get trapped with drawing non-cycle cards. Of the test games I have played I am 5:1 where my only loss was because i went 5 turns with only 1 mana. It does have its weaknesses but for a budget legacy deck I am pleased with it