Not sure how these would fit with the complications of CoS, but...
One route is the Collection Quest. Opening the portal requires a ritual with X number of ritual foci, all of which are unique objects of mystical significance (though not necessarily innate power). There's a list of X+Y potential foci. The PCs get clued in by investigating the series of thefts and robberies and are able to reverse engineer the list and its use, or maybe thwart a theft and get info from the thieves. Then it's a race to collect objects themselves and deny the lich the magic number needed. Though obviously that's just a delaying tactic until they can defeat the lich itself.
Another route is Keymaster and Gatekeeper. This is like a scaled down version of Collection Quest, where there's just two or three vital pieces needed but they're harder to get. Stuff like "The unbeating heart of Strahd von Zarovich" and "The birthplace of the mortal woman who became the Raven Queen". Less travel and investigation, more big fights over key items or information.
Yet another is Hit The Books. This is where you roll back the villain's progress so that they don't know how to achieve their goal either, and the point of contention is their attempt to learn how such a ritual might be attempted at all. At this point the PCs are more likely to trip over enemy agents as they scour the planes for tomes of forbidden knowledge or attempt alliances with dark figures in return for their knowledge. This might actually work best with the CoS framework, where you don't want to distract the party too much from what's already going on in Ravenloft.
