2. Fanart where the character is the opposite gender from canon.
3. Partial genderswitch happens almost exclusively to male characters, when they, as part of the plot, gain some female parts (boobs, vagina, womb) but also retain some male parts (penis, testicles).
4. One of the devices used to make M-preg somewhat plausible.
5. A hell of a lot of fun to write.
I drew genderswitched Ed Elric, because he's pretty enough to be a girl so why not take it the rest of the way.