Honestly, Nika and/or Joyboy seem like decent candidates. Imu could be afraid of them because they got access to their power without any restrictions being placed on them. I don't think that's likely, it seems like Nika probably just has similar powers due to both likely being "Gods". But, it's something that feels possible.
Potentially, ALL Devil Fruit users, if Imu does turn out to be the Forest God who created the Devil Fruits. The Devil Fruits were probably never meant to be released into the wider world. I feel like it's pretty likely that Nika/Joyboy lead some sort of "prison break" that lead to a bunch of Devil Fruits to be scattered all over the world. So, all known Devil Fruits could be "unsigned contracts" with Imu, of a sort.
I am keeping my eye on Scopper. Chopper should be treating him pretty soon. So, it would be kind of interesting if Scopper had a Abyss Mark tattoo somewhere on his body, that only gets revealed when Chopper treats his wounds. Maybe only someone with a Abyss Mark can injure someone else with a Abyss Mark? Maybe that could be the secret of the anti-regeneration technique? Of course, we have seen similar powers in the past, like Miss Goldenweek's Color Trap, and the Marys' all-seeing eye charms on Wano. So, maybe it could have something to do with Imu, or maybe it could be related to the power system, but not quite the same as Imu's Abyss Marks. Or maybe it's not happening at all, which is honestly the most likely outcome.
MAYBE Vander Decken the First? His story is basically the Flying Dutchman's story. It COULD be explained by a deal with Imu.
Similarly, The Man Marked By Flames. If only because we know so little about him. He's got a black ship. Maybe it's black due to the Abyss? I feel it's unlikely, but it's a idea.
Mihawk...is a maybe. It would explain the ringed eyes. The problem is, we really don't know anything about Mihawk, so who knows if this actually makes sense. Again, I feel it's unlikely. But, not impossible, I guess.
Some people have pointed out that Dragon's face tattoo could be a Abyss Mark. I find this one HIGHLY unlikely. But, I can't say it's impossible, either.
Other than those, I can't think of many people Imu would have had a chance to interact with. There aren't really that many people we know about who lived hundreds of years ago. Or would have had a chance to even meet Imu recently. The Gorosei and the Giants are the only ones who have lived anywhere close to that long. There just aren't that many characters who could turn out to be it.
