Quote from MikeCline83 »Notion Thief has been surprisingly good in every deck I've put him in; I think you should try him out.
I agree that Nivix Guildmage needs to go; Dual Casting could be a good alternative as it's cheaper and repeatable each turn (more if you can unto your creature). Increasing Vengeance could be awesome as well.
Guttersnipe might be good; since you're casting a ton of spells, even though it doesn't trigger off of copies.
I kindof really like Thoughtpicker Witch. Oona, Queen of the Fae could help with the sacking plus she's just an amazing card.
Wrexial, the Risen Deep should also be in here despite his high cc since the cards hit the GY after you cast them so you can cast them again