"He is the Chosen one!" (just have some mystical sort announce this somewhere to one of the characters with people gaping-maybe at the town fair or a magic show?)
"Evil Twin" (clone or add in the stolen at birth cliche-PC gets accused of awful things only to find out he has an evil twin. Could add in mysterious injuries that are being done to the twin for triple cliche goodness)
"Mustached villain for Marriage" Capture a woman, tie her to the rails and threaten her if she doesn't agree to marry (you can just make him looking to get her inheiritance, or he could be bonkers and completely obsessive)
"Find the McGuffin!" Think Belgariad. Super awesome powerful artifact that can destroy the world gets stolen, chase the bad guys until big climactic fight. Bonus if the McGuffin is a sentient goodly being that can't actually save itself, double bonus if it can only be used by "one pure of heart", triple if it has to be from a certain bloodline, quad if the evil villain is actually *gasp* the true parents of the hero/s)
"Disturbed native burial ground" nasty things start happening, can also be retyped to read "curse of xxx's tomb" and involve an archaeology class/tour group/oops, I fell into an ancient tomb in downtown new england.
Amnesia. You think everything is X, but it's really Y. Bonus points if it involves superhuman powers, double if it's from top secret gov't programs or aliens, triple if the head of the group is a parent, or they're the alien's unknown heir to the galactic throne.
Portals! As above, only everyone finds out they're really lost people from <portal land> (think harry potter)
Ancient extinct race is not really extinct, only in hiding. In fact, you're all descendants! bonus if someone's a lost royal or chosen one or prodigy of some lost ability/tech.
Science fair robot/computer/computer program gains sentience!
Time travel. Pick a show/movie
BBEG is going to detonate a nuke in <populated area> you have 24 hours to find him and disarm it.
Zombies!
Vampires! (pick your flavor, but double cliche if twilight-with bonus scaling for each session as a pain threshold exercise, triple if taken stright from dracula)
Frankenstein! (seriously there a ton of varieties to choose from)
Werewolves! (see above)
(Any of this can be reflavored to mundane if need be, just "Science!" it. That whole "any sufficiently advanced technology is virtually indistinguishable from magic" )