It's a bit of a genre breaker for a D&D game, I know, but I was going to ahve my players be set up by the villains and catapulted forwards in time. The cliffhanger of my hiatus will be them realising they are 20 years or so in the future, and the BBEG they have only vaguely heard of has taken over. Fun to be had as they meet up with older NPCs, discover how they "failed" by vanishing, and then when they get back to the present day they have a clear reason to fight the BBEG, because they know know what will happen if they don't.