I'm actually working on this very project, from two different angles. 
First, Ranma and Akane make the same wish, at the same time, on the same
falling star after the failed wedding.  They end up repeating the events
of their first meeting, with some significant changes.

Ranma's tongue works faster than his mind, almost.  This clues Akane in
that Ranma may not be who she thinks he is.  When they confirm the
situation, Kasumi or Nabiki (depending on which draft of the draft),
walk in at the most embarrassing time (a passionate girl to girl kiss.)

The other take, is Nodoka is given two wishes by Urd.  One wish is
permitted by the heavens in accordance to the policies we all love.  The
second wish is granted to special people who perform great services for
the gods.  In Nodoka's case, she was Urd's teacher and "earthly mother."

As a result of both wishes, R & A & N are granted a second chance, with
their memories intact.  They slowly realize this, and continue building
their relationships, even with Genma trying to hide Ranma.

I may drop Nodoka's aspect, but I'm not sure.  Just thought you should
know, there is someone playing with this idea.  And the credit goes to
Dave Eddy and Lord Archive for their inspiring works.

Bryan Neef