People have made the comment that Sandbox games are harder to play because without Quests, there's no direction.
I don't subscribe to that thinking. I think a good Sandbox game would simply change the way you get "quests."
Instead of going to some NPC and picking up a rinse-and-repeat quest, you pick up your quests by playing the game.
You hear NPCs say that something happened or they need a certain resource, that's a quest.
That NPC may even be a "town crier" (UO had them).
You go to a dungeon, and see a wall carving with either stories or distinctive images, that's a quest.
Rewards would be included in the drops and chests, etc.
There are loads of ways for a game World to set in motion things, quests, for players to do. And adding in new stuff into the existing world can keep such a game very interesting for a very long time.