Since we aren't told who Iori has a child with(or what his daughters name even is) I wanted to share my headcanon on the matter.
Now, I imagine that this is not Iori's biologically child, but his adopted daughter(i also headcanon iori as asexual but that doesn't have to be a headcanon for iori to adopt a child). I believe that during his work as a defense attorney, Iori hears about a case where a young girls father was killed and after the person who murdered them was prosecuted the mother just couldn't care for her(either due to her own trauma, mental state, or not having enough cash and no one to help her) and put her up for adoption.
After learning of this, Iori obviously feels some connection to the little girl, since he lost his own father young but he was lucky enough to have a mother and grandfather that could care for him. So Iori goes to see the girl, he wasn't planning on every having a child because he knew they required a lot of care, but he just found himself falling in love with the girl. Maybe it was because she still seemed so bright even after what happened, or maybe he felt a kinship to her because they lives were similar.
Either way, Iori goes through the process of adoption and adopts the girl into his family, where he feels happy he can be the father he never had. Sure, he had his grandfather to care for him, but he passed and Iori always wondered what it would be like to have a father. Well, now he could be one.
You might have noticed that this headcanon doesn't really center around Iori having a romantic partner and that's a part of my headcanon. I don't ship Iori with anyone, but I do imagine Iori has a queerplatonic relationship with Takeru so I headcanon it would be both their daughter but if you ship Iori with someone else(chosen child or not) or ship with no one this headcanon could still apply.
I just really like the idea of Iori adopting the unnamed little girl we see in the epilogue and her having same trauma of losing her father so Iori could be the father he never has(what can I say, i'm a sucker for these types of tropes and no it totally doesn't say anything about me)
