Lore consistency is something they’d have to manage, for sure.
I don’t think tier sets are the only type of new loot permitted. My suggestion is to add specific BiS across different content so that you spread out what people want to run, and to add context-specific BiS.
Imagine a new raid where mobs have very high armor value, such that melee initially has a hard time doing good dps. Now further imagine that a lot of gear drops that defeats this within the raid. Finally, imagine that there are a few generic BiS items also in the raid. People will want the BiS items. In order to get it, they’ll likely need the armor pen items, so they’ll have to run the raid to gear up for it. At the same time, getting through the raid does improve their overall ability (since they have a few new BiS items), but doesn’t dramatically invalidate the older stuff, since the new BiS items are limited to a few slots.