Actually a palliative care nurse has been quite successful in answering this very question!
Bronnie Ware was an Australian nurse who rose to fame through her blog Inspiration and Chai, which was all about collecting the regrets and "dying euphonies" of the elderly patients in her care.
She then turned the blog into a best-selling book, listing the 'top 5' regrets that people had on their death bed:
1. I wish I'd had the courage to live a life true to myself - not the life others expected me to live.
2. I wish I hadn't worked so hard.
3. I wish I'd had the courage to express my feelings.
4. I wish I had stayed in touch with my friends.
5. I wish that I had let myself be happier.
I really think we can learn a lot from the concepts encapsulated in these 5 points, and I hope anyone that reads this answer thinks about what they could do to make their life more fulfilling.