Imma try to use my brain to see who the Skeleton is by trying to address every option.
Ally: The easiest one to debunk. The Reaper isn't an ally because of the simple fact that he tried to actively hit Zoro.
And thus; Enemy: The best option, which enemy? Well I'm glad that you asked.
Beast Pirate: I highly doubt that such a remarkable Beast Pirate would be hiding for so much time. And we know that he isn't part of the main parts of the crew because both the F6 and the All-Stars are full. Plus the fact that the Beast Pirates use animal fruits.
Big Mom Ps: Like, do I even need to say anything here? I guess I should. First of all, her crew isn't even in Wano, and second of all, we never saw any character that could be the potential user of such a mythical fruit during WCI, add that on top of the fact that they tend to use fruits related with food and that kind of stuff and the answer is there.
CP0: I also saw that it might be a CP0 agent, this might sound alright but it isn't imo. First of all, why would a CP0 go out of his way to capture Zoro? Yeah, I know that he is now a rather easy target, but the Gorosei literally said "fuck everything else, we only want Robin". So any CP0 going against what the Gorosei said should be crossed out of the list. That's honestly the biggest argument that I have, but it's so remarkable that it probably solos this option.
Death itself: It's the most credible option here. With Chopper being worried for Zoro's state and with Zoro basically sleeping surrounded by his own blood. Zoro is close to death and thus, the Shinigami is coming after him.
Extra: How in the hell is Enma's embodiment? That Skeleton literally attacked Zoro and to top it all, Death isn't the King of Hell.