Coming up with original ideas is difficult. You might be putting too much pressure on yourself trying to come up with 3 original ideas. One un-original way to be original would be looking to heavily twist the invited context of the stimulus.
Remember they will be focusing more on how you show your knowledge of the concept of belonging, and how to convey it as a writer then your ability to write a magnificent story based on the stimulus.
Maybe you could try including belonging on several levels in the story, so you can illustrate clearly how connections create belonging, and can be between individuals, or the global world.
For example, a cruddy story which does this could involve a group of criminals, who you explore as characters, someone new comes along and you develop the story with this new character attempting to fit into the group (you can mould this however you like). This can illustrate the connections between people. Then move on to how the group doesn't belong to the world, policemen that want them 'in jail where they belong'. Personal and social belonging being the key aspects there, you could link in cultural belonging somehow as well.
Just keep in mind as you write stories what you are trying to show the marker. If you have the ability to show the marker that you understand belonging on a high level, then originality and the ability to blow the marker away with your fantastic writing and ideas become less important. Not everyone can write an amazing and original story. That being said, also keep in mind that sometimes the ability to convey belonging on a high level requires writing that shows thought, so remember to use lots of techniques such as extended metaphors, etc.
