I am a big fan of this idea, but I'd say it should use the bulitien boards found in adventure guilds and in other areas. I like the idea to create my own, that would take some work, but would be fun.
But the overall idea of going up, accepting the hunt, finding the mob, and killing it I'm all for. I also support the tiered structure. It wouldn't be based off of levels, it would be based off of what you have already completed. But I'm also for this system to take it's time and there being a large number of hunts to keep people occupied and not complete them all right away.
Also, it would be nice to have some hunts open up only after other quests, leves, sidequests have been completed. Just to keep it interesting.
