In addition to these, I am also considering Wriggling Grub but haven't decided what to cut.
EDIT - A week later, I decided to swap out Zulaport Cutthroat for Vengeful Bloodwitch. They are functionally the same in a duel, while I can put the Cutthroat to better use in Commander, where it is strictly better. Plus, there's the full art version of Bloodwitch, which I am adding as I swap to the full art Swiftfoot Boots.
Grimdancer is an excellent, flexible card. But I already run Vampire of the Dire Moon, Gifted Aetherborn, and Vampire Nighthawk, which all share design space with Grimdancer. Most of the time, you're going to choose either menace/deathtouch or lifelink/deathtouch, in which case, the Nighthawk is just better. Replacing either of the other two would increase my average cmc, which i try to avoid without good reason.