Last weekend I watched John Carpenter's The Thing. It's pretty good.
http://en.wikipedia.org/wiki/The_Thing_(1982_film)It's about a shapeshifting alien that can take over anyone it infects, disguise itself as anyone it's assimilated, and turn into all manner of horrifying things that kill you.
Then I read about how vampires in DF will be infiltrating your fort, disguised as a dwarf, killing people off while you try to figure out who it is.
Adding something like the Thing to DF would take vampires to a whole new level, and I think this next update has the tools to do that.
It should be able to:
-Disguise itself as anyone it's killed (People or animals) or as whatever it looked like before being infected
-Shapeshift into a more dangerous form if it's discovered, like growing extra claws, getting bigger. If it's not trying to disguise itself it will grow whatever body parts are convenient
-Animate it's severed body parts and the body parts of those it kills, which become other Things. (So people's heads can grow legs and crawl away)
-Infect people (though biting, body fluids etc) causing them to turn into Things even if they survive.
I think at least some of these could be accomplished, with all the new things in the next patch. (vampire and werecreature transformations, the new undead, necromancy and other interactions.) I'm guessing it would be a pain to mod in though, if it's even possible.
It would be pretty overpowered but extremely Fun.