So its basically a bounty mission. There are terminals already that offer heroic quests and other missions similar to what you are suggesting.
However, its not player made, and its not repeatable, or as repeatable as you are suggesting it to be.
The problem is you have a list of secret mission that trigger an effect in the persistent world and that boss is open to everyone, and a lot of people would have access to that mission meaning some will take away from others or they can do the same quest. If they take repeatable bosses from other players that means you have to make a lot of bosses, spawn randomly, which is still fun and doable, and if not and they are shared then that is the same as the terminal box for quests.
So being able to do them repeatedly with different bosses in different locations sounds fun, but still a bit of work to set up and offer enough that it is varied. Also if the boss is not evident in the persistent world, but is activated to be unlocked, that takes away from the appeal of a bounty in the persistent world and is more of an instanced fight in the open world such as a heroic for example.
I like however the idea of bosses, and mini bosses, and it being repeatable. I think its a nice idea, and should also involve player made content as well. And would like to see how it can apply player made content.
Thanks for the feedback. You make a good point, and I instantly though about making it instanced, to which you made another good point...not as appealing if it isnt in the open.
A solution to this would be to have it spawn when you get close. This way it cannot be farmed by other players (at least not easily) but would still be open world. Of course some of them could also be instanced, some could even use mobs already placed in the world...you simply get a bonus for going after them directly.
There are plenty of mobs and bosses already on the maps.