Alot of options basicly, so you have a choice on what to focus in the game, different ways to get things or experience. For example besides normal quests, alot different NPC quests, little quests part of a story in the map, tasks (can ofc be from an NPC too), possibility to get unique things only a few people have, etc.
All kinds of goals a player can reach. But all in a way that it's an option, not that a person is forced to do anything to keep up with the rest, but that it's still worth to do. Also don't make things to complicated, to much options such as all kinds of systems can also work confusing in a way that a player doesn't know enough about it or simply doesn't know what to do.
An other thing I always liked, a world that is not to big, and every place has it's meaning, so every place is worth to explore. This can be with new quests, parts of others quests or simply other unique things what makes it worth to go there. For the rest, giving a story to the map itself also makes it more fun. Names to cities, mountains, rivers, forests, other kinds of areas, making areas property of a city and for example make this a part of a story/quest.