I had an idea for this, why not use the open world as the Eureka/Bozja fill-in?
The first step could work like this:
• Instead of Tomestones, the turn-in items cost Bicolor Gemstones (from FATEs).
• Each of the EW areas has its own turn-in item, six total. They will appear in the Bicolor Gemstone vendors in that area once you unlock the Relic quest.
• You need a combination of 3 turn-in items that changes depending on what weapon you are aiming for.
• To spice things up, we'll bring in multiple Critical Engagements (FATEs but with limited members) to each area as well with Combatant's Priority Deployment (i.e. those who help spawn the Critical Engagement get spawn in priority).
• They could also add Manderville Lockboxes/Caches that you can earn from EW FATEs once you unlocked the relic quest.

With this people would get their open world content, open world FATEs will have a purpose (outside of the rare, big FATEs that give you turn-ins for glam items), it won't be too hard or different from current gameplay, and it won't be as annoying as ARR's relic FATE grinding. *(*´∀｀*)☆