A few good ones that I haven't seen mentioned here:

http://www.fanfiction.net/s/3939897/1/ - "The Name's Tendo Akane, And I Don't Lose". Ranma has been in a coma since the Neko-Ken. Akane has been training seriously and is almost at canon-Ranma's level. And canon-Ranma has just made a Wish on the Nanban Mirror. Oops.

http://www.fanfiction.net/s/3566724/1/ - "The Dark Lords of Nerima" - Ranma and Ryouga get mistaken as the latest evil villains... by both the Senshi and the Youma. Oops.

http://www.fanfiction.net/s/220883/1/ - "Martial Artists & Mayhem, Third Edition" - Ranma 1/2 as set in a generic D&D world. The deities are Ascended main cast from other series, including Slayers and Tenchi Muyo. Akane is an acolyte of Ryoko's church. Kuno fancies himself a Paladin of Tenchi. Oops.

--
Of all the tyrannies, a tyranny exercised for the good of its victims may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end, for they do so with the approval of their own conscience.
-- C.S. Lewis