Well out of that list the ones who make the most sense (to me) are Helena and Nik.
Helena has been reduced to a evil shadow in Nik's life and she had been hanging around for too long with the same agenda since she arrived. GET CONTROL OF THE CASSADINE HEIR!!! If she dies, let's face no-one will really miss her.
Nik well it wasn't too long ago that he had prime-time deal in the works. If I remember correctly HIS contract was scheduled to end in January (I think). Maybe he has gotten another primetime offer or decided to look for greener pastures (again). If it is Nik who is being killed off because of Tyler's contract I wish they would just recast him. Their is a shortage of available men in PC we can't afford to let one die off!
All this said, I vote for HELENA to join the ranks of the (un)dead!!!
You know that on this show DEAD does not mean gone forever!