Soap Opera week. (Cloud has a brain tumor! Sephiroth realizes his mother is Spanish! Jecht and Squall break up... again! Yuna plots to marry Edgar and steal him away from Faris!)
Random Gender Swap Week.
Yaoi Week, and then Yuri Week.
OOC Week. (Act completely OOC, you may not be anything resembling canon at all.)
Mpreg Week. (Sephiroth is pregnant, again.)
Mary Sue Week. (Your character must fall in love with a Mary Sue/Marty Stu temporarily.)
Implausible Relations Day (Everyone is somehow related to everyone else.)
AOL Speak Day (You must use the AOL translator. If you don't need the AOL translator... we don't want to know.)
Purple Prose Day (only long poses squeezed full of the largest words are acceptable)
One liner day (you may pose no more than one line of text)
Half-Esper week (Everyone becomes a half-esper. EVERYONE.)
And... I'll go surfing through Fanfic.net later to get more ideas.