Perhaps the brother and sister get separated due to some misunderstanding. Years pass and they both try to move on but deep down they are always sad. In the end, they find out about the misunderstanding too late when one of them is on their deathbed, and they can't make up for the lost time.
One possible ending could be that the grandpa's family finally realizes how much they've neglected him. They start visiting him regularly, and he slowly starts to regain his old happy self.
The wolf could sacrifice itself to save another animal or even a human who once tried to harm it. This selfless act would be a very sad but also heroic ending to the story.
One possible ending could be that the fruitcake was thrown away in despair, ending up in the trash.
Perhaps the sad caterpillar's situation gets worse. It fails to find enough food or is finally caught by a predator. This would be a tragic ending for the poor caterpillar. However, its story could also serve as a lesson about the harshness of nature for other small creatures.
The ending could be that the doge's owner comes back and realizes how much the doge has missed them. So, they decide to never leave the doge alone for such a long time again and give it lots of love and attention.
Perhaps the stray cat finally finds a small corner to hide in as it gets weaker and weaker from lack of food and illness. It curls up and closes its eyes for the last time, with no one to mourn its passing or even know it had ever existed.
One possible ending could be that they never get to see each other again. Jack fades away into his own world and Elsa is left in her ice - filled kingdom, both with a sense of longing and loss.
Another possible ending could be a life of eternal loneliness. For example, in some stories where a person loses their loved ones due to war or disaster and has to live the rest of their lives in solitude, carrying the pain and memories forever. It's like a wound that never heals.
One possible ending could be that the pigeon just gives up. It becomes too weak from hunger and loneliness and passes away in a quiet corner.
One possible ending could be that another kind player comes across the sad dog and decides to take it in as their own. The dog then gets a new home and is happy again.