My game is an open-world. But i'm stuck on the goal/objective. Is there a list of goals/objective for an open-world games?
This is a good question, and I know a group of people who have addressed this question in the following link; in the video, some of the ways in which games fulfill the player's desire has been discussed in general terms.
There are a number of needs that games fulfill, an open-world game may fulfill the following depending on the content, theme and modes of play:
1) sense of discover: if your game focuses on content and has an attractive landscape, players may derive pleasure in discovering new things, in which case, you can place set-pieces like ancient ruins that tells a story, and the player is trying to find all the ruins to reveal the story, all the while roaming a landscape that is pleasant to look at and move through.
2) fantasy: if your game allows the player to be more than a normal human being and someone/something the player would like to be, such as a fish or a bird or some mythical creature, than the player will enjoy the role playing as the avatar. For such games, I suggest a story-driven series of objectives like fighting enemies, finding items, finding locations etc., since in such games the important part is the role-playing, the goals the player try to fulfill may not need to be significant in themselves, so long as they are consistent with the theme of the protagonist and the world.
3) narrative: if your game is set in a world where the player instinctively feel curious, then you can prepare a compelling story that resonate with the theme. For example, if your world is set in a vast desert with debris of spaceships scattered about, the player will want to know who were in the spaceships, why have they fallen, who is the protagonist etc. In narrative-focused games the objectives should not be too complex or involve too much repetition, since these may either become unnecessarily frustrating or distracting.