I think it'd be cool to have a dedicated max level subzone for each zone, ala Warlords, that could be scaled and tuned similar to M+, with rewards to match. You could even throw in some procedurally generated content there to make it even more fun. Imagine if each zone of a new expansion had a combined mini Torghast and M+ dungeon like experience built somewhere into it.
However for the rest of a zone I do agree being able to come back and curbstomp leveling mobs is an appropriate award for sufficient gearing at max level.