I'm not so sure Lefevre is really a Sagan fan... He doesn't really like him that much. Plus, he'd be competing in classics with plenty of Belgians at Leferve's disposal... I think Quickstep is a no go for Sagan. The only way he would end up there is if he accepted below, way below, his market value contract. I don't see it happening, even Sagan isn't that stupid...
Btw, that's an interesting idea him competing in MTB for Specialized... Plus, he was always fond of the States, and California in particular. Said many times, he'd love to move over there... He has huge fanbase in the U.S., he could easily find a nice career there... But after his divorce and limited access to his son such move might not be in his cards anymore... We'll know soon... I'm guessing about this time next year, we'll know