Dan Henderson is Open to UFC Return

Of course there are fighters outside of the UFC but every fighter of any significance is now under Zuffa. As for Overeem and co defecting to Japan that's just speculation on my part. Its highly unlikely as DREAM's president has already gone on record saying FEG/DREAM and the K-1 are dead.
In regards to Henderson knowing something prior to deal who truly knows but id hazard an educated guess that he didn't. Hendo was only responding to the Zuffa buyout of his contract and the implications on himself.

 

There's an article in mmabay about how Overeem is exited at the news of the aquisition and Werdum has also expressed interest in returning to the"octagon" . I think most fighters will like the news but i'm sure M1 isn't 1 of them.