The only card I can see is dark impostor, although that i just from personal experience. I've included him in several decks, but I've never really gotten much use out of him. If he works for you then that's my only switch suggestion.

I would say take out Dark Impostor in exchange for Liliana, Death's Majesty. His activated ability is way too expensive, and though he seems like he might be a cool card, he is too expensive to actually use. I've played a few games against him, and he looks scary, but that's the problem. Once you play him he'll become a target and will be destroyed before you actually get to use the ability.

That's just my opinion, have you been able to use his ability to a good extent?

Usually Dark Impostor Caries his own weight, I'll always make it to 6 mana and a 3 mana creature that demands removal Is pretty useful. I was looking more at Oona or Koshuko

Two other cards I think you might also find use for would be Bonehoard and Nighthowler. Especially in larger games, these cards can one shot a player if they aren't countered. Play a creature first, then play and re-equip or bestow for an extremely painful mill, and both have durability to creature destruction. Both can be equipped to an existing creature for a surprising blow or played alone to draw out counters or kill spells.

My personal recommendation to remove would be, as i said previously, Dark Impostor as I've only very rarely actually used his ability because he's often killed before I get any use out of him. He lacks durability and unless you have 9 mana or it's a small game he will likely be dead before you have the six to throw at him, or at least that's my experience. I do agree with Barsan that Oona, Queen of the Fae or Kokusho, the Evening Star are both cards that aren't as functional in the deck as others however, and you may wish to find alternatives. if they work for you, more power to you, but Kokusho especially doesn't really play to this decks theme. If imposter is in place for removal there are better options.