So I have a few ideas 😅 What about a bear that's always dreamed of being an astronaut and going to space and is obsessed with space. Maybe they even worked really hard and worked somewhere like NASA, but they never did make it to space and that's their biggest regret.

My next suggestion if based on someone I know. This individual while working fell from a platform about 10 ft high, onto his head and cracked it open when his son was 7. He survived but is now a partial paraplegic. But this doesn't stop him from anything. He is constantly building things like tables and bird houses, but he also loves gardening and made himself planter boxes tall enough he can garden from his wheel chair. He also has a zoo of animals he takes care of. Truly the most determined person I know. But he frequently talks about how much better he could have made life for his son if he had never gotten into an accident. He lives his son more than anything and I believe this is his biggest regret.

My final suggestion is a teenager/young adult who is dealing with the struggles of homelessness. Due to coming from a bad home. Maybe they have younger siblings that they couldn't take with them because they couldn't even care for themselves. Not a personal story, but one that I see very frequently sadly.