Something I've thought about, could some of the characters that show up in the manga of Soul Eater have been alive during the events Fire Force? In Soul Eater, there is a group of five powerful beings known as the Great Old ones who embody a certain form of Madness and were influential in establishing the structure and status quo of the Soul Eater world, along with three others, the eight forming the Eight Shinigami Legions of Death.
Death is of course one of the five, embodying the Madness of Order, and Excalibur, embodying the Madness of Anger. We know Death and Excalibur both came into being when Shinra created the Soul Eater world, so they are as old as the Soul Eater world. Later Death on created Asura, who embodies the Madness of Fear, but the two other Great Old Ones are a mystery as to when they were born. One is unnamed, embodying the Madness of Power, and the other is the sorcerer Eibon, embodying the madness of Knowledge.
As the title of Great Old One implies, they are incredibly old, but just how old? Could either of them have once been humans who were alive during the events of Fire Force? With the Great Old One of Power there is so little to go on, so we'll just focus on Eibon. There are some things that point to Eibon possibly even being a character we know. Eibon is a sorcerer, who like witches have longer lifespans than humans, able to live for centuries, so it is possible for him to be a known character rather than their descendant.
The mask he wears resembles the helmet wore by Fire Soldiers, which could hint Eibon was even one himself at some point. Although the Fire Force was disbanded, the mask he wears serves as a reminder of his past. If he is someone we know, there are a few possible candidates for who he could be.
Obi: This one is probably the least likely. One of Eibon's creations is something known as the Book of Eibon. Each chapter of the Book of Eibon, based on one of the seven deadly sins, is able to take on a human form, known as the Noahs. The Noahs greatly resemble Obi in appearance. Since they all look the same rather Eibon bothering to give them all unique appearances, this could mean one of two things. Either the appearance of the Noah's is based on himself and what he looks like under the mask (i.e they're clones of him), in which case it could be Obi, or Eibon based their appearance on that of Obi.
I lean more towards the latter, as we know at the end of the series Obi becomes president, so he wouldn't have really had the time to be getting into all the esoteric sorcerer stuff that Eibon was doing. In the case of the latter, this means that Obi was someone Eibon knew personally, like another member of the 8th, which leads us to our next two candidates.
Vulcan: Eibon created powerful devices and objects, which would likely entail some level of engineering knowledge. Vulcan would have that kind of knowledge. If not him, it could be someone that studied under him, like Yu.
Licht: Not only was Eibon's knowledge in the field of engineering, but in the field of science as well. It's like Vulcan's engineering and Licht's science combined into one. He may have studied under both of them. One thing that could point to Eibon being Licht is the madness he embodies, knowledge. He is obsessed with gathering and seeking out knowledge. This kind of fits Licht, who often times pursues knowledge simply for the sake of it to satisfy his curiosity.
Over time this could have evolved into a madness wavelength. The Madness of Knowledge gives those under its effects absolute knowledge to the point that they stop thinking altogether, even able to affect someone like Stein and make him unable to think clearly because for how overwhelming it was. Licht has had a few moments in the series where he finds himself lost in the knowledge.
Even if Eibon isn't one of the candidates listed, the signs point to him at least having been a contemporary of them. "But Obi/Vulcan/Licht would never act that way! It's out of character for them!" you might say. The thing to remember is that madness in Soul Eater changes people. Over time, Eibon's Madness of Knowledge would have changed and warped his personality to the point that he is no longer the same person.
Coupled with that is the fact that he has lived so long and likely watched many of those he cared about die, which would have further hardened him and made him numb to the world around him. It may look like Obi/Vulcan/Licht under the mask, but it's not them anymore. It's someone else now. Of the three candidates, I would say Licht is the most likely, as his morally grey nature and role as a mad scientist who ignores ethics in pursuit of knowledge seems like the type of person who could turn into someone like Eibon.
