1) Catspaw
The PCs are hired by a baatezu, in the guise of a righteous human or an
upper planar even, to assassinate a particularly infamous baatezu lord.
The one who hired them is really just the next in line to inherit the
position.

2) What?  (Or, Charge of the Dark Brigade)
This adventure starts out in the middle; given the number of creatures or
situations which can affect memories, have the characters start imprisoned
in a fiends home plane, with their last memory being in Sigil, safe and
sound (or perhaps, being in Sigil and told to go visit someone); the
possibilities are endless; perhaps the characters were sent on an important
mission for the forces of good, and now they have to figure out not only
WHAT they were sent to do, but HOW to do it, or maybe the baatezu are
claiming they signed contracts (particularly good for all-lawful parties)
agreeing to do a mission for them involving the Blood War.
My current idea has the players start in a mobile battle fortress of the
baatezu, after apparently having signed up for a suicide mission against
the tanar'ri in the Abyss (hence the second title, Charge of the Dark
Brigade), however this sort of adventure could have infinite abilities
(or be used to start off new players to the campaign; they may be planars,
but they don't remember much other than what fragments the DM allows,
and crawling out of the river styx, barely alive.

3) The No-Book
An extremely powerful, secret artifact.  Anyone who's name is written in
it is erased from all history; no one knows them, their deeds have gone
undone (if they weren't of much consequence) or are done by someone else.
The only catch is, they're still alive.  The only people who remember them
are others erased by the No-book, and the person who erased them.  
The PCs are either approached by an 'acquaintence' who knows a lot about
them, but they've never heard of him, or are erased themselves (or perhaps,
are erased following trying to help the acquaintence)
(I can think of one obvious solution for the party)

4) The Greatest Escape
The characters are trapped on a section of a plane (most likely Carceri), 
via an immature conduit.  There is a town in this section but little else.
No gates to anywhere provide a means to escape.
The only way to escape is to change the mentality of the town enough to
push it into another plane, with them along for the ride.  Easier said than 
done, of course.  (This leads to a whole bunch of fun, as the characters may 
have no luck converting the citizens to a good alignment, and instead may
try to slip the part of the plane into another evil plane, and then have
to escape from there.