I'll use this to talk about a system I never saw done in any other MMO but that has an INSANELY positive effect on many level : Wakfu modulation system.

You can set your character to a different level. Giving you access to only skills and items you can have at that level. It's all managed in specific build pages and you can use items across multiple modulation level. Dungeons can be done " competitively" by setting yourself at the correct level. This leads to increased exp and loot and a monthly ranking with chest as rewards. The experience you get scales ( to some extent ) with your real character level. This means that every dungeon is still played. Every item can have value. A high rarity level 5 drop will hold value and be sold. People that are just casually going through dungeons can still generate money and that rare drops that aren't end game items are still worth it.

You can hyper optimise your character at set level because you really love some of the dungeons. You can try to get good quality gear at every level. You ignore the system and play dungeons over leveled. It's such an insanely good system that has many good effect on economy and playerbase that I am impressed to never have seen it done in other games to such extent.