I'm not convinced he'll go...
For a start there's the matter of who can afford him? Forget who doesn't have a sprinter, etc. - this is the most important consideration. OPQ is being bandied around because at the Tour de France, Cav's former HTC DS (and good friend) Brian Holm started telling everyone Cav was unhappy. Brian Holm is of course a DS at OPQ.
Secondly, would Sky's plans have changed that much from when he signed for them to the Tour? Since their inception the team have stated that they were going for GC. OK there was some PR stuff at the start of this year about going for the Green jersey too but the way they lined up for, and the way they rode, Paris-Nice in March showed how they were going to tackle the Tour.
Cav won three stages at this year's Tour and was unlucky to crash in the finale of another - and we know from experience that if he's in the mix at that point he'll probably win. It's a measure of his quality that this is seen as failure in some way. Of course if the target is the Green jersey, then I suppose it is. In my opinion it's the changes to the points competition that have partly undone him at Sky as a team racing for GC cannot chase down breaks or delay a break getting away just to pick up the intermediate points. Which is ironic as the changes were designed specifically to enable a serial stage winner like Cav to win Green.
I don't think he has to move to win stages at grand tours or have another crack at Milan-San Remo, or races like Paris-Tours. But if he wants to move to win the Green jersey again, then he's going to have to find a team dedicated to him with the quality of riders that were at HTC. Then he has to hope Sagan isn't riding.