Assuming that "boss" is specific to the main-arc antagonist.
Alvida - Currently with Buggy's crew, nothing points to that changing
Morgan - Last we saw him, he was drifting in the open sea
Buggy - More probable since he's actually with Luffy currently, he is however, gaining a lot of followers and is not likely to give his new found fame up
Kuro - Who even knows were he is
Krieg - Gin would be more likely to join then him, so no
Arlong - Of all bosses so far he was probably biggest asshole, Nami forgave Hatchan, but i doubt she'll do the same for Arlong
Smoker - He would have to leave the marines, unlikely that he and tashigi will seperate
Mr. 3 - Possible, but he seems more likely to join Buggy
Crocodile - He also want to be PK, so it would be odd for him to join
Enel - Ummm, he's on the moon. Though it would be possible for him to come back in some sort of twist
Foxy - Ha Ha, no
Lucci - With the rest of CP9, no reason for him to leave
Moria - That would certainly be interesting, don't see it happening though.
Kuma - Unless he gets his mind back, no
Hancock - Has often been speculated as a possibility, most likely of the boss characters to join due to her one-sided love affair with luffy
Magellan - This would go against everything Magellan did, so not likely