If you want your rare items - I'll gladly take a HNM that spawns once a week and randomly attacks an area. Either there are some people close by who can man up and kill it, or everyone wipes...so sad. Lower the drop rate to 1% and by the end of a year we might have one or two of the item floating around the world. And if "hardcore" folks want something to do, you can spend your time hunting these.

There are lots of ways to create rare items that don't involve sitting in a circle waiting on a pop. This might have been exciting when the functionality and capability at a dev teams control were more limited, but today it's just boring. You can have monsters that are not frequently found in the game, or that are hard to get to, and that have very low drop rates, or challenging conditions to spawn. Those are all at least interesting game mechanics. Your proposed solution is just boring gaming and not something that most of this community has an interest in dev cycles being wasted on.