Okay, how about this:
A joint Starfleet/Klingon mission[...]It can be set either after Nemesis, or in the years after the Kitomer Accords, which, considering the subject matter, probably makes more sense.
Excellent idea! And that would also be around the time Klingons got their ridges, a secret that was never lifted and that might interest quite a few fans.
Personally, I think it was an unfortunate accident with a chemical or biological weapon that led to permanent DNA changes. A Romulan weapon, of course. Klingons would never use weapons that kill children and other civillians - it'd be the paramount of dishonour.