How about a serial killer who is killing off people who he/she thinks is a "sinner"... he/she doesn't have to be right-wing, Catholic, what-have-you, but goes on a rampage based on his/her moral concepts, however correct they may be. (NOT like what happened in Ghost Town, which was just someone going vigilante to "protect" his family, but someone who's convinced that they're some sort of archangel with the power to punish sinners. And depending on what "morals" this archangel has, in Vegas, there would definitely be LOTS of options for sinners/victims.) He/She seems entirely normal on the outside, going about the day like any other businessman or whatever, which at first throws the CSIs off. His/Her MO could be a religious symbol of some sort, whether Pagan or Christian or otherwise: a crucifix, a pentacle, an ankh, or some other symbol that stands for redemption...

As for which CSI is targeted...Catherine certainly has a colorful history, she might be targeted as a victim...I would say Sara (if only to get Grissom back in the picture for a little bit! :D), but I think she's had enough from the Miniature Killer...but my personal favorite would be Wendy, to show even more Hodges/Wendy on the show!

Who would play this character? Erm. Call me crazy, but I can totally see John Barrowman (Torchwood, Doctor Who) as someone who looks perfectly, UTTERLY normal (if a bit gregarious) on the outside but who could totally change directions and harbor insane beliefs about his power over others. (And I REALLY want to see him on American TV, LOL.) But barring that, Ewan McGregor would make another cool, calm, collected psychopath if the character is male...I personally think a female character would be interesting (even if the Miniature Killer was female, that's usually not expected, serial killers being a woman and all that), but I've got nothing on possible actresses. :/