You know, there's something most of the prior replies have wound up missing.
This, at least to my admittedly untrained eye, is a crossover, and our pro-
and antagonist from the future are none other than Zero and... well, I THINK
it's Sigma... of Rockman X4 fame.

That said, a whole new can of worms gets opened up. The standard "hero" and
"villain" roles need not apply, necessarily. Consider...

Somewhere in the far future, the original source code for the Sigma virus
has been found in the work files of a certain doctor, who was experimenting
with artificial intelligence. His lineage was traced back as far as possible
(or perhaps the time-travel device could only work at a certain minimum
interval) and ended up at Ranma Saotome and Akane Tendo. (Sigma, if you'll
recall, is the Greek letter for S. Perhaps the doctor labeled the project
files with his own initial or some such.) Now...

Zero has been put through the wringer by Sigma, physically and emotionally,
so far as Reploids have emotions. He decides to head back in time and break
up Ranma and Akane so the Sigma virus never comes into existence.
Unbeknownst to anyone, Sigma's already hijacked a prototype humaniform
mechanoid and headed through the portal; the brief prologue was about Cain
and X watching Zero head through.

So...

What Sigma was doing (or at least trying to do) was brainwash Akane into
falling desperately in love with Ranma. Take Shampoo at her most clingy and
up her a few orders of magnitude; that's the effect virus-boy was going for.
He'll probably try and strike at Ukyo or Shampoo, or perhaps Kodachi, but
Akane was the least likely to present problems... and the least likely
person anyone would notice a change in.

Now...

Zero, after keeping Akane's company for a bit, (carefully NOT revealing why
he's come back) is faced with a bit of a dilemma. Is he willing to deny two
people a chance at love to remove a threat that has always been contained?
Or will the threat still be there even if Ranma and Akane DON'T marry? You
can never tell exactly what in the past affects the future...

Now, I'll grant this is just off the top of my head, but this fanfic has the
potential to be a lot more than pure action. Although... weren't a few of
Takahashi's stories just extended fight scenes?

--G. Falconar