Seeing Villeneuve at Renault would definately be something cool.
Another maybe door could be at Williams. Frank Williams never said that he wouldn't take him back if he needed to.
But anyways, none of his doors in Formula One are really decided yet. I hope he stays in F1 and finds a good wheel. Wouldn't be all that great to see him go. He'll probably enter DTM or such a league like many other F1 drivers did (Alesi, Frentzen, Hakkinen, all good fellows) and also participate in the prestigious LeMans race.
I'd wouldn't see Villeneuve in NASCAR though. Doesn't seem to be his type of driving, and observing his current situation, it would be close to impossible. He just married, his wife is waiting for a baby, and I don't think he move to the US, travel around the country from Febuary to November for something like 36 weekends. And NASCAR will have how many cars next year? 63? So many people are talking of getting into NASCAR.
Anyways, I hope he'll find a good spot to remain in Formula One... But a good European league would probably also suit him if his hopes are lost.