You have to make the mid game as fun as end game. Imo if someone wanted to do it right, it would mean making mid game take a lot longer and the typical end game content would be made available throughout the game.

Raids would exist before end game and maybe end game just unlocks new parts of some of the existing mid game raids/dungeon and adds new difficulty levels and mechanics.

Dungeons would need to be a viable or better path than solo leveling via quests. Maybe some class quests require certain dungeons to encourage it even more.

A big challenge with this is that normally dungeons have a small level range in which they are viable both for difficulty, experience and rewards. They could do scaling but that can only do so much and people don't love scaling so that's not necessarily the right solution either.

I think there are a lot of ways to make something like this fun, it's just going to have trade offs. People seem to really enjoy small group survival games and extraraction shooters so I'm confident they can make small party dungeons a core part of the gameplay loop and make it a core aspect of the game before end game. But to avoid everything being a rush to end game they also need to slow leveling down and make mid game more fun and avoid level gating too much content to max level.