It would be nice to have some clarity about this. I've always been told that if a multi-stage is a physical container it should adhere to the distance rule just as any regular cache.
Have you read the knowledge books? My own physical multi stages can be 5 feet from each other or across planet Earth, as long as they are all at least .1 miles from everyone else's physical hides.
None of this applies to nonphysical or virtual stages or "hides" such as virtuals, earthcaches, or multis with nonphysical stages.
I think what is more shocking, to me at least, is the number of instances I have read about large numbers of caches, such as geo art or power trails, being on someone else's property without permission.
I love the idea, but Keystone is correct. Third party must be reviewers, with strict guidelines. Not every plaque is worthy of being a cache. We also don't want HistoryCaches (or whatever they would be called) outnumbering caches with physical containers.
Please no. Puzzles shouldn't be difficult for the sake of being difficult. They should be clever and give the silver a sense of satisfaction when he or she has that aha! moment.