My ideal is to work until age 99.

Because I plan to work for such a long time, my requirement for work is to be able to go to work excitedly humming a song every day, enjoying the process.
Additionally, due to the long cycle, I want my work to have compound interest effects over time. This way, no matter how small the task, consistently working in the right direction will eventually lead to growth and strength.
My goal for ages 30-40 is to build such a work platform for myself.