It would be cool if there was level 60 dungeon content that expanded on low level dungeons. For example razorfen, bfd, wc, sfk, zul furrak, ulduman, sm, mara, etc that had a section with level 50-60 mobs, drops, bosses.
Like, new portals in an existing dungeon hub area that go to a new 50-60 section. Or maybe even a instance portal within the dungeon itself that goes to another instance if that's possible. The sense of mystery would be pretty cool while leveling. You're level 40, you complete a dungeon, you see something deeper. Dang, can't wait to come back to that later on.
Or perhaps level 50-60 "heroic versions" of dungeons. Maybe empowered or corrupt versions of old bosses with new mechanics mixed in with some new stuff. Resurrected by the power of the old gods influence or something to fit the theme of the server lol. Or maybe a "corrupt/bad" timeline of old dungeons, relating to the caverns of time or something, as there are plans to expand that area.
Bring some variety since people run the same few dungeons at 60.
Adding unique drops to those dungeons/also more to existing 60 dungeons that lack cool epics is also nice. Since those are a big reason people rerun dungeons. "Lets run scholo for flask, lets run black morass/hfq for the trinket, lets do emp runs for ironfoe"
Same with old zones. For example, wetlands (a level 20 zone) has some level 60 dragon mobs at the gate of grim batol. Maybe add some more 60 sub-areas around Azeroth. Add some quests or whatever. Also more high level rare mobs in lower level zones. Like the firstborn of argual in gilneas. With some cool drops.
This would be a nice way to add on 60 content without having to make whole new dungeons/zone themes (which is of course great as well) and expand on the world of azeroth. It would be fun to go back to some of these places at 60 more. A big thing with classic wow is making the whole world feel alive, versus just moving on to the next thing over and over like later expansions do.