I have an idea of a content that does not have an upper difficulty limit. Just have a content, it can be a dungeon or a raid, and you can set the difficulty from 1 to 100 or 1000 or 10000 or whatever. There's no upper limit. The devs don't have to test it because it's not meant to be beatable on release. Maybe players can beat maximum difficulty level 5 or 6 on release.

On the next even patch when the max ilevel goes up, players can start beating difficulty level 7 and 8. Then on the next even patch players can beat difficulty level 9 and 10. Beyond level 10 is not beatable until the next expansion. There is no level sync. When the next expansion is released, players can beat difficulty level 15 or 16.