That would be pretty interesting. Just talk to an NPC to start certain fates like we do now, and just your party challenged the monster if you jump in during the time frame. That would also solve the issues with creatures zoning in and out. Yoshi should really look into this one Yagrush. ha
They could also bring back the idea of HNM to an extent using this idea. If you had a certain "rare" item you could go to a location and enter an instance with your party for a chance to obtain some sweet gear.