So your players got themselves a shiny new copy of the PHB2, and they are dying to try it out… great! But maybe the problem you are facing as a DM though, is that you’ve got your story on track, the PC’s involved are part of a greater tale within the world you’ve crafted, and you’ve got visions of your BBEG getting pummeled by the characters you’ve been playing with for almost a year now. Not a problem for the creative DM… introduce an adjacent storyline!
This could be an interesting experiment. Every other game session, the party can play the alternate characters they were dying to roll up out of the PHB2, and you can devise an alternate storyline, withing your world, just for them. It can even take your plot in directions you weren’t even imagining.
Let’s say the original party, Party A, was being sent by The Council of Elders on their missions. The Council of Elders fears that the Wizards of the Broken Tower, a primordial magic using cabal of wizards will one day gather enough power to take over the world. The Wizards need the Amulet of the Abyss to finish a ritual that will grant them this power. Maybe Party B can be hired by the cabal, (under fake assumption) to find the Amulet in the Abyss. This could be Party B’s 30th level goal, they (the characters, not the players) believe they are recovering the Amulet to keep it from being used for evil). In the end the parties both realize the threat the world faces and you can craft an ending to your campaign that has them both be heroic and save the world from a catastrophic primordial uprising or something. Party A has to do this… and Party B this… maybe characters can even cross parties.
Perhaps you guys have been dying to run paragon or epic tier adventures since your main guys are still in heroic. Well, here’s your chance. The stuff that maybe you expected to happen “off-camera” can now happen with an alternate party!
Another idea is to let in on parts of the the plot one of your players who has been dying to sit behind the DM screen, so he can run Party B through the alternate quest. You get to pick up a character sheet and play while still pushing your story along. Everyone wins! He may even come up with stuff you wouldn’t have thought of.
I’d love to know if any of you more experienced DM’s have ever run something like this before, and your experiences with it!