Heroes - Elle against Sylar, Peter or Claire. It would obviously be seasons 1-3 (I know she didn't show up till 2, but I have done some exploring of her life before season 2 in the past).
Glee - Rachel against Jesse, Finn or both.
Glee - Santana against Puck.
Glee - Quinn against Puck, Finn or both.
Vampire Diaries - Elena or Katherine against Stefan, Damon or both. This one I could do bookverse or showverse, but I prefer book.
Veronica Mars - I would be willing to play either Lilly or Veronica in a fab four psl (Lilly/Veronica/Duncan/Logan). Obviously pre-season 1.
I have others in my journal here, but these are the ones I want the most.