How do you feel about exploring other planes/times/multiverses? Can your campaign support a onetime(?) event which throws the characters somewhere else giving you time to figure out where the adventure is going as the characters struggle to figure out where they are and how to get back to where this quest is. Given that the big bad might have magical powers he could use those to do it, someone allied to him could, or even the mists of Ravenloft could. Once somewhere else you could dangle plot hooks to get them to stay there as long as you want them to, and when and if they come back from wherever they went to, the look on the BBEG's face will awesome.