I'm thinking of writing a short and simple Ranma Fanfic, but I'm stopped
by the thought that it either must of been done before or is so obvious
that it need not even be attempted.

You see, I'd simply make clear the central theme of Ranma: "What's so
special about Akane, what's the one crucial thing she offers Ranma that
none of the other girls do."

It would be an ordinary morning at the School of Indiscriminate
Grappling, the occupants have dwindled down to the married couple that
owns and operates the school.  Akane asks Ranma "Why me?" and over the
next several days he replies with a series of questions that at first
really tear her up, but as she works out the answers she agrees with him
that he made the very best choice.  And, at long last, her mind is set
at ease.

It's so short, so simple, so obvious, that somebody's already done it,
and I simply haven't found it.