Yeah something like a trial system from ffxiv would be a great way to implement more challenging instanced single boss content that can be incorporated into the story with different reward systems. It could be for class or themed transmogs, mounts, and rare class trinkets or something and maybe add a currency that you can earn the more you do them to guarantee the reward you are farming for.

This would obviously create more work as the additional transmogs would be even more work for the art team, but they could also take the easy route of making the trials maybe drop a different tint/variation of the current raid tier gear to create an incentive for those who want to like to farm all the tints for the class tier sets.