So, as you may recall (if you watch the show -- if you don't, you have probably cleverly avoided clicking on the cut-tag), last season ended with Beckett in a state of wedding interruptus watching Castle's car burning merrily in a ditch after it was pursued by an ominous looking black SUV.
We know from the promos (and the title of the show) that Castle is not actually inside the car, where he would already have been burned to a crisp. (Although watching Beckett and Alexis solve murders together would certainly be different...) Given that Beckett and Castle really ticked off the mob in the season 6 finale, the natural assumption is that this was a mob hit.
Except that the expected body is not in the car. And the mob is not particularly subtle, so there's no reason to believe that they would pull Castle out and leave his car to burn. This would require someone who is more devious.
Senator Bracken might come to mind, but he is pretty safely in jail and is probably suddenly lacking the resources to pull this off now.
There is, however, someone who was dusted off and put back on the shelf earlier in season 6, a murderer who likes to play with his food and torment his victims.
That would be Jerry Tyson, the 3XK Killer.
I think it's him.
I suppose I'll find out next week. :)