Hmm.. if you play
Nim Deathmantle just because of intimidate, you could also swap it for
Trailblazer's Boots. More evasive (who plays only with basic lands in EDH except mono decks?!) and cheaper to equip.
Coalition Relic could also be interesting for you, since you can abuse it for more mana with proliferate.
Spark Double could also be VERY interesting for your deck, since its cheap and allows you to put a second Atraxa on the battlefield.
Depending on how satisfied you are with your land base, you could try to replace the 3 color lands (e.g.
Arcane Sanctum) with storage lands (e.g.
Calciform Pools). They are a little weaker in colorfixing, but once you're able to proliferate, they'll get interesting. Speaking about the land base, in WAR they printed a very potent land for your deck:
Karn's Bastion.