One thing that the Buffy and Angel writers seem to do is if they're writing themselves into a cliche, to reverse it or produce a twist on it.
For example...the entire world is getting brainwashed against Fred, including all her friends, and she comes across this seemingly friendly demon who seems to be the only being in the world isn't brainwashed. So, they'll join forces and fight back, right? Nope, the demon tries to eat her. Another example...Cordelia and Buffy are both going for Homecoming Queen and the announcement is that it's a joint win. So they both got it, right? Nope, it's too other girls.
How could you apply the cliche reversal to the Rod of Seven Parts/Assemble the McGuffin scenario? Some random ideas:
* Instead of retrieving the parts, the intact mcguffin needs to be busted apart and it's pieces hidden by the PCs to stop Bad Stuff happening. There are opposing forces trying to put it together, and the PCs may have to hunt them down and hide a few bits again, and do it better this time.
* The PCs are being misled by false prophecies planted by the Forces of Darkness (TM), but in the course of their searches they gradually uncover evidence that putting the mcguffin together is going to cause the cataclysm, not stop it.
* There are two different cataclysms, and another party (perhaps of another race, like aquatic elves) is collecting bits of the mcguffin for their own needs. When they meet, will the parties fight or argue over whose cataclysm is most important, and needs the mcguffin the most?