A) hot horror chicks. Everyone, women included, are huge fans of pale faced dark haired vampire-esque girls. Take morticia or Elvira for example. Try going to suicidegirls.com also for ideas.
B) LOTS of detail. No one wants JUST text and a little logo. If youre going to spending your hard earned money on a tshirt, you want a good, high quality image. for ideas, check out http://www.toxictoons.com/. They're a good example of scenery and character designs
c) REAL LIFE. Try taking photos of your own bloodbath done at home. Like my avatar for example. that is from a movie of course, but if you could re-create something super gore and offensive on a tshirt, that would be AWESOME.
Ill add more if I can think of it