We did have something like that in Chimera/Hydra until they moved it to DF. Although it is more instanced than open world. There are some technical issues with it though. What if you have multiple groups engaging the same boss? Would u have multiple Chimera's on the same map? And if you make it so that only 1 can be engaged at any 1 time, then u'll have issues with ppl spawn camping and holding the boss and not letting others get in. On a side note, Sagolli desert sounds like the perfect place for Ruby Weapon. Once (and if) airships become craftable, we can engage the extreme version of Ultima Weapon in mid air.