This is one of my favourite modifiers on a TG/TF scenario, more specifically, a reversible triggered change applied worldwide to a large fraction of the population.
Probably my favourite fanfic ever plays out the scenario of Ranma catching a cold and passing it on. A cold Washuu (from Tenchi Muyo) has been playing with. This eventually results in effectively a planetwide spring of drowned girl/boy curse. It also features look in crossovers with Futaba-kun change, evangelion and a bunch more.
The author's website went down a couple years ago. Looks like there's a backup halfway down the page here:
http://www.rakhal.com/FFIndex/ranlstT9.htm