About 2 years after I started working, I finished paying off my student loans and I started saving about 40% of my pay. I always thought I would retire at 50 or 55. Then in 2014, I got fired from my super stressful job. I had given everything, and I realized that I was done. Luckily, a few months earlier, i had found MMM and realized that I didn't need to wait until 50. At that time, when I thought of retirement, I thought of traveling, learning languages, taking some interesting classes, being in better shape, etc. In the three journey, I rediscovered my passion for dancing, and decided that the first couple of years of FIRE would be dedicated to that. As time has progressed, the plan has become more refined. Now I plan on spending most of my time taking care of my body, so cooking healthy meals, practicing yoga, practicing dance, taking classes, going out dancing. At the same time, I realized that my grandmother is old now, and I will be spending a lot more time with her while I still have the chance.
All this is to say, that you can have a vague idea to start, and fill it in as you realize what is important to you.