Whatever plot you/we decide on, my interest has been piqued. :)
I'd prefer either scenario 1 or 2, but 1 probably gives more opportunity for thrilling moments, as murders may happen as the plot advances.

[quote=@BassDropp]
If it was a serial killer, then it'd add the component of multiple killings and a sense of urgency to figure out who s/he is. May I suggest if it's a serial killer that s/he kills the victims with a signature or some sort of pattern. i.e. They find the woman's body shot in the head but mutilated to make a smiley face on her body. Like maybe there are cuts shaped like X's on each breast and then gutted to make a smiley face. Gross but it'd be interesting to say the least.  
[/quote]

This reminds me of The Bridge (Bron/Broen) season 3... :p