The player gets to decide, specific motivation not required; the journey is the motivation.
But you still need gameplay mechanics to support the user's activities.
If the user is motivated by quests, you need good quests.
If the user is motivated by conflict, then you need good fighting mechanics, etc.
If the user is motivated by building/construction, you need strong mechanics to support that.
Unfortunately, I suspect you'll find very few user's who are motivated over the long term by just wandering around seeing cool stuff.