Idk how you're going to serialize this? Like the whole setup is created for a one time joke.
- Surprise, she's actually your osananajimi who stopped developing mentally from 4th grade
- Whoopsie the entire family being fed upon that as a misunderstanding.
And then what? Author prolly gonna be forced to dip into your usual second girl joining along with lame drama. The way I see it, the only serialization is in the form of web comics with usual ≤5 pages each chapter.
You say that as if there wasn't like a hundred mangas who started with a simple joke and either blow it up out of proportions, or derailed the plot entirely. There are also usually changes between the oneshot and the first chapter of the serialized manga, which can introduce more concepts to the plot.
If it was up to me, I would make it about them trying to remain acting "mature" when they are at school, which usually involves not being together, but they miss each other so they try to find opportunities to meet up in secret, where they can help but being childish. Bit by bit, they progress in their romance, they get to meet other people that are hiding their interests because they are seen as childish, and so on. A bit of drama could be added by exploring her past in America, she says she didn't interact with others because she didn't knew the language, but that could be a lie she uses to hide a traumatic moment in her life, because there she also had to act mature, not to fit with everyone, but to avoid being bullied or something like that.
