Another idea is a story of unrequited love. A boy is in love with a girl who is completely unaware of his feelings. He watches her from afar, always there for her when she needs help but never getting the love in return. He sees her fall in love with someone else and has to deal with the pain of his unspoken love and the knowledge that he will never be with her.
A family that is torn apart by financial problems. The father loses his job, and they start losing their home. There are constant arguments between the parents because of the stress, and the children feel helpless and scared. They have to leave their friends and the familiar neighborhood behind, and the family unit that was once strong starts to crumble.
A sad love story could be about a couple who were childhood sweethearts. They made all kinds of plans for their future together. But then, one of them gets into an accident and loses their memory. They no longer remember their relationship, and the other person has to watch as the person they love becomes like a stranger. They try to make the loved one remember, but it's all in vain.
Here's an idea. A man and a woman work in the same office. They start a secret relationship as office romances are frowned upon. But then, the woman gets a great job opportunity in another office of the company far away. She decides to take it for her career growth. The man is heartbroken as he can't leave his sick mother behind to go with her. Their love slowly fades away as they get caught up in their separate lives.
'Love Unreturned: A Heart's Sorrow'. This title straightforwardly tells the reader that the love in the story is unrequited and that there is sorrow involved.
'The Shattered Bond: A Tale of Lost Friendship'. This title clearly conveys the main theme of a broken friendship. It gives a sense of something that was once whole and strong but is now in pieces.
A fanfic could show Link being sad after seeing the corrupted forms of the Champions. He remembers them as they were in their pure forms, and seeing them as corrupted spirits or in some other twisted state could break his heart. He might try everything in his power to free them from their corrupted state, and his sadness could fuel his determination in the story.
A story about a sick pet. A little girl has a beloved dog that has been with her since she was a baby. The dog gets very ill suddenly. The girl tries everything she can, spends all her savings on the vet, but in the end, the dog still passes away. She is left heartbroken, and every time she sees the empty dog bed, she is filled with sorrow.