You could repeat certain key events/encounters from earlier in your campaign, while changing the intentions/abilities of the key NPCs (both allies and villians), the conflicts consequences, etc. Or show what would have happened if the heroes failed in a specific quest.
Maybe the villain has the same general objective as before, but this time there's a good reason to allow for his success. What effects would this have when the PCs return to their own reality? How do they justify tampering with one reality in order to perhaps influence their own native one?
Shift the alignment of EVERY known NPC by one factor (i.e LG becomes CG, etc.) let the players interact with these alternate personages to discover their new agendas (or same agenda approached via a perhaps radically different viewpoint).
Perhaps the plane is highly morphic -- responding to every inhabitant's (frequently conflicting) desires. The environment could be continually changing based on these intentions. Introduce changes (at least) every single session. Some based on what you know the players would like, others based on the wishes of others, and have the players:
1. Determine WTF is going on.
2. Who are the major influences in this shifting reality.
3. Figure out how to subtly shift the intentions of those individuals having the most profound influence...
You could allow (knowledgeable) players to expend action points, XP, etc, in to alter specific details of the adventure for any single game session (no promise of a lasting effect beyond that). Hell, you could even have only a single player "in on the secret", allowing him to intentionally manipulate the reality (perhaps using the mechanics above). Or reverse the situation and one player left out of the loop (without explicit knowledge of the mechanics, or the ability to alter events -- while the other players have a bit of fun messing with him/her...
Obviously some of these ideas would require a rather dedicated, mature, and imaginative group of players... oh well, back to plan B.
