I'm personally still leaning towards female.
That's mostly because I still suspect Imu isn't the villain of this story. A lot like Loki, Oda's just being a little too blatant about painting Imu as a villain. So much so, that it probably isn't the case. Oda loves a twist. And if Oda's going THIS hard with the Devil imagry, I'm expecting a big twist where Imu's actually a pretty reasonable person.
And so far...Imu actually has been acting fairly reasonable? Imu's whole thing has been about making deals, which implies some level of give and take. Imu's been TRYING to conquer Elbaph without spilling blood. The God's Knights refer to Imu as "ontai", which implies that they're pretty fond of their leader, which implies Imu treats the people close to them well. And Imu cowering in fear of Joyboy's Haki Knot during Egghead just doesn't seem like something you would do for a big bad you're still trying to build up. Oda seems to be hinting that Imu isn't all bad.
Lulusia got wiped off the map, sure. But, that's really the worst thing they've done so far. And there may be bigger-picture reasons for why Imu needed another hole in the ocean, or to take a bunch of lives. It might have something to do with the nature of Devil Fruits, or Demon Kings, or both. More on that line of thought over here:
https://worstgen.alwaysdata.net/for...ncident-was-a-singularity.73794/#post-6378324
And if Imu isn't the villain of the story, then I think it'd be easier for the audience to forgive Imu if they're a girl. That's not fair, but that's the way it goes. A girl who does wrong things is more easily forgiven in media, than a guy who does those same things. Oda has a history of redeeming nearly all his villainous female characters, after all. The groundwork is already set up for Imu to get a pretty easy path to redemption, if Imu turns out to be a girl.
But, also, you could say that Oda has already foreshadowed that Imu is actually female. At least a little bit.
Imu may be a parallel to Miss Goldenweek.
Miss Goldenweek has a form of "Symbol Magic" similar to Imu's Abyss Marks, in the form of the Color Trap. More on all that over here:
https://worstgen.alwaysdata.net/forum/threads/imu-still-may-not-be-the-sea-god.76012/#post-6482582
But, the most important part takes place during a cover story.
During the "Meet Baroque Works" cover story, we see MIss Goldenweek use her powers to alter her friends. Pretty similarly to how Imu wants to change the World. And how Imu changes people with the Contracts and Marks. Making their dreams come true in different ways.
(Chapter 404, 1085)
Everyone gets new forms. Kind of like the Gorosei's Yokai forms. Or Harald's change in personality.
(Chapter 406-407)
Everyone in these two cover pages COULD be seen as a reference to the Gorosei and Imu.
Drophy/Miss Merry Christmas becomes a Princess. That could make her a parallel to V Nusjuro as the Warrior God of Finance. Someone who gets to be in charge of the money.
Zala/Miss Doublefinger becomes a Barista. That could make her a parallel to...lets say Warcury as the Warrior God of Justice. (Which I'll fully admit, is the weakest parallel here.) Someone who listens to her customers problems, and provides solutions. A "bartender" is...kind of like a "judge"?
Mikita/Miss Valentine becomes a Chocolatier. That could make her a parallel to Mars as the Warrior God of Environment. (Which, again, I'll admit is pretty weak.) Someone who can turn nature into something beautiful. Thus, the chocolate house she's holding.
Gem/Mr 5 becomes a Firefighter. That could make him a parallel to Saturn as the Warrior God of Science and Defense. Someone who uses science to save people's lives.
Babe/Mr 4 becomes a Pizza Deliveryman. That could make him a parallel to Ju Peter as the Warrior God of Agriculture. Someone who helps feed people.
Lassoo the dog-gun becomes a Tank. That could make him a parallel to the Ancient Weapon Uranos. A weapon that helps defend everyone.
Marianee/Miss Goldenweek becomes a Artist. This could make her a parallel to Imu as the God of this World. Someone who can change how people see themselves, and the world around them.
And those aren't the only ones who are changed.
(Chapter 408)
These two didn't join Miss Goldenweek. And were left in prison instead.
Crocodile becomes the Pirate King. That could make him a parallel to Joyboy, the First Pirate. Someone who wants to be free, and didn't go along with "The Artist"'s plans.
And Daz Bones/Mr 1 becomes a Hero. That could make him a parallel to Davy Jones. Someone who wants to save people, but got turned into a supervillain by others, instead. A hero, that got turned into a boogeyman.
The two who got left behind, also parallel the two who most likely opposed Imu. Implying that, at one point, Imu and Joyboy and Davy Jones may have all been "friends". Until they split up and chose different paths.
It all kind of works! You have to squint. But, it kind of works!
(Chapter 409, 908)
And then everyone who joined with Miss Goldenweek moves to a deserted spot in order to create a cafe of their own, where they live happily ever after. They built the Spider's Cafe.
Which parallels pretty nicely with Imu and the Gorosei all living in their "ideal place" on top of the world in Mariejois. They built Pangaea Castle.
They both made ideal homes, with people that they cared about. And helped make their dreams come true.
Imu and the Gorosei seem VERY likely to be dark mirror images of Miss Goldenweek and Baroque Works. So, if Baroque Works are former villains who turned good. Maybe Imu and the Gorosei are former heroes who turned villainous over the course of hundreds of years?
Miss Goldenweek becomes a artist...with a mustache. A little girl...pretending to be a man.
Could Imu be doing the same thing?
There are examples in different media of a female being raised as a male to rule in a traditionally male role. Princess Knight. The Rose of Versailles. The Marvelous Land of Oz. Real-life pirate Mary Read.
https://tvtropes.org/pmwiki/pmwiki.php/Main/RaisedAsTheOppositeGender
https://en.wikipedia.org/wiki/Mary_Read
That way Imu would be a girl, but still explain the "-sei" honorific. Imu is a girl, forced to pretend to be a man to maintain her rule.
I'll admit, it's not a perfect one to one. All the Gorosei are male, while some of the Baroque Works/Spiders Cafe staff are female. Imu doesn't necessarily NEED to be female, to still make this parallel work.
But, I do think Oda was hinting at something with all this. It feels like there's something here. Maybe I'm seeing connections where there are none. But, it's all lines up a little too well to be a coincidence.
Either way, Imu and Miss Goldenweek are likely parallels for each other. Does that mean Imu is also a "little girl"? I say...MAYBE! I'd at least say, the foundation is here.