I'd give your guess the top odds if I were running a Vegas casino.
The supposition about avoiding retaliation is correct, I believe. But I doubt France would be the one.
France seems to be some kind of "safe house" for the terrorists. They stage through there, they have planning sessions there. But they never conduct operations there.
It would appear that there is some sort of tacit understanding between the terrorists and the French authorities, a quid pro quo -- a mutual "don't hassle us, we won't hassle you" policy.
Great Britain, Spain, Italy, Australia...any U.S. ally in the WOT would be asking for trouble, too.
Germany? Maybe. Canada...???
We pretty well know what the U.S. response to a "dirty bomb" in Toronto would be. But would Sheikh Abdul bin al-Wahabi know?
He does know, however, than security arrangements in Canada are a joke. That there is no lack of terrorist cells for local support. And that there is absolutely no danger of Canada retaliating. Moreover, Canada is next door to the The Great Satan -- as if to say, "You're next".
The RCMP needs to have its antennae fully extended.
The Libyans blew up a french airliner back in the 1980's. There was a thwarted plan to crash a plane into the Eiffel tower, and the Ried shoe-bombing plot was against a plane leaving Paris.
What would it likely be?