For those familiar with FFXI, they had these instanced boss fights that dropped valuable gear and items known as BCNM (Burning Circle Notorious Monster). More difficult fights with greater rewards were in ISNM and KSNM. As you fought normal mobs out in the open world, they had a chance to drop beastman currency which could be traded for orbs that granted access to specific fights.

I envision something similar to this in hopes (1) add\ new content in the same spirit as Aquapolis of PotD and, especially important, (2) make the open world feel relevant and revitalized. This involves introducing a beastman-like currency that drops from FATEs where the value is commensurate to FATE difficulty. Then we exchange currency corresponding (untradeable) items used to gain access to special and difficult trials (which could feature unique and interesting mechanics beyond just the boss itself). Each fight would have treasure commensurate with its difficulty, but unique to these fights.