I don't have a cohesive theory for the end scene, but below is a list of subtle "clues" that could hint to what it might be:
- On page 19, the dad mentions "The promise of Room 8." Arata responds, "We theorized it was an entrance to another world", to which the father responds, "Is that so?" I'm not exactly sure what it means, but I think that was foreshadowing something.
- On page 21, the dad speaks to the mom's picture about Arata, saying "At that time I was drawn to him, I'm glad we chose Arata. My eyes were right, weren't they?" This must be a reference to when they adopted him. Suggests they "chose" him for a specific task.
- In Chapter 1, Arata wonders "If there is another future where he doesn't have these five sisters". He then ponders on "fate" deciding one's life for them, such as their siblings, and their spouse.
- In Chapter 2, they ask Banri what she remembers, since she was 4 when Kotono was adopted. She suddenly remembers something, but then refuses to share what she remembered.
- In Chapter 3, the dad is looking at the picture of the mom. In the picture, mom is holding a baby. Baby has same hair color as Banri. Then, right at that moment, Banri enters the room. Dad says "Of course it's you, Banri."
My guess is that Banri is not actually adopted, but is the actual biological child, and Arata was chosen to be her "soulmate". My guess is that the dad explained everything to Banri when she entered his office in Chapter 3. This could explain why she's so insistent on taking him out on dates and flirting with him.
On the other hand, there are many references to Ouka:
The kind of heels that Ouka most often wears.
The daughter’s outfit is literally Ouka’s outfit copied from one of the chapters.
The wife’s posture: Ouka has the habit of keeping her hands behind her back.
The scene with the wife is similar to Ouka’s first confession.
In later chapters we see little Ouka, and if you look closely, Arata’s daughter is almost a copy of little Ouka.
However, I believe that every theory should be approached with distance, because Reiji probably didn’t have anyone specific in mind, he just placed a scene from the future so that readers would make theories, something like what SEO did in “Megami no Café,” where he mentioned that when showing the daughter, he didn’t have any specific girl in mind at that time.