I’m thinking proteus saff in a creatureleas deck. Use it on niv, he comes back out, you stack your deck with whatever cards you need in the top 10, his etb triggers and you get them into hand.
Also, for backup, in case something happens and your combo gets countered or milled off the top after you've set it up, run Mizzix's Mastery to cast your full combo straight out of your graveyard.
And how about the flavor?! a Niv-Mizzet combo deck that wins with EPIC EXPERIMENT! Come on!
Or, how about this! You use that combo to put an Epic Experiment into your hand amongst other things, AND set up your next 5-6 cards for an immediate game win via Epic Experiment into a bunch of copy instant/sorcery cards like Bonus Round, Doublecast, Howl of the Horde, or Reverberate, etc into a card that if copied a million times will just win you the game like Chandra's Ignition, Breath of Malfegor, or Glimpse the Unthinkable.
Also, for backup, in case something happens and your combo gets countered or milled off the top after you've set it up, run Mizzix's Mastery to cast your full combo straight out of your graveyard.
And how about the flavor?! a Niv-Mizzet combo deck that wins with EPIC EXPERIMENT! Come on!