HOLY CRAP TL;DR! 8D SPAM IS DELICIOUS... (but crack is better)
There seems to be a dearth of suggestions. *FIXES THIS!* Yeah, also, re-raising a few of the older suggestions, because they're too awesome to ignore. Consider this a vote in their favor.
The Stranger*: How does your character wish he or she was different? What would they change about themselves? Let's let them be the person they wish they were, for a day. Well we all have a face / That we hide away forever / And we take them out and / Show ourselves / When everyone has gone / Some are satin some are steel / Some are silk and some are leather / They're the faces of the stranger / But we love to try them on
Urban Sprawl: Welcome to New York, New York! It seems the Big Apple has been transplanted to the world of CU— much to the displeasure and alarm of the locals. The New Yorkers, that is, not CU. So here, folks: have 8 million disgruntled city folk dropped on your doorstep. Good luck with that. Do you know the city, though? Then you might notice how it's not quite right. Are you from another world entirely? Strangely enough, you might find you recognize some things.... Maybe it's 'cause the city that never sleeps landed right on Emptiness....
The Island of Doctor Moreau*: Each character is transformed into an animal.
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Genderswap*: Well. This is awkward.
See No Evil, Hear No Evil, Speak No Evil: Characters wake up only to discover that they are either blind, deaf, or dumb.
Seven Deadly Sins: [Option 1) Seven days, seven sins. For each day of one week; characters are subjected to the 7 sins. Example: On Monday (envy) everyone would be envious of each other. (Pride) everyone's extremely proud. (Sloth) everyone's lazy. etc etc. [Option 2*) Grab-bag of sin— Lasts three days, not a full week. The morning of the first day, your character is presented with seven scrolls: pride, covetousness, lust, anger, gluttony, envy, and sloth. Until one of these scrolls is chosen, the character may not leave the room. Whatever sin they pick is theirs to exemplify for the day. On the second day, there will again be seven scrolls. On the third, fate chooses for them.
Love At First Sight: Fall deeply in love with the first person you see. Stalking encouraged.
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*Also, suggesting a staggering system for some of these days (possible for use w/ any of the days w/ *asterisks*):
For instance, for a day like Island of Doctor Moreau:
Transformations would last 2 days, and not everyone is an animal at the same time. For instance: On Monday and Tuesday, Reno is human, but Rufus is a fluffy bunny. On Tuesday, Yako turns into a labrador, and stays that way until Wednesday night. On Wednesday morning, Rufus turns human again, Yako is still a lab, and Reno turns into a pigeon. Reno is a pigeon until Thursday night. By Friday, Rufus, Reno, and Yako are all human again, but Bert, who turned into a mouse Thursday morning, only turns back into a human Friday night.