I love the Storied Realms bosses, they were all engaging and had unique mechanics. I'd love to see them appear in higher tiers in some way, Vault seems like the easiest answer.
This would also allow some of their materials to be available at higher tiers, and open more mid-to-high-end options for Crafting for the current stage of the game.
This would probably break the lore since Storied Realm bosses are specific to the story.
Maybe the Difficulty slider can be unlocked to be unlimited instead of having a difficulty cap. Then you can just sleep to respawn the boss in the story and go fight them again at a higher difficulty.