Hmmn. I'm working on a Wherigo right now that incorporates several waymarks. Here's how it works: the player is a would-be thief out to steal a priceless sculpture. In the game, I have the player wander around six different outdoor sculptures (each of which is a waymark). In the end.. well, I'm still working on the "final act."
Anyway, it seemed like a fun way to combine all three Groundspeak games. And yes, there will be a related geocache for this Wherigo. There almost always is, at least in this part of the world.
Here's another thought: what if this fell into the game category, kind of like the Lucky 7? Instead of listing waymarks from various categories, the game (called something like "Wherigo Wheriwas") could list waymarks used in the cartridge/experience. Just a notion, I haven't given this much all that much thought. ;-) Touchstone may have ideas as well, since he's got a Waymarking-based Wherigo in Monterey, CA.