"Motivation for what?" is the question. If you've got enough money, properly invested, such that you don't need another job, then, this is not the question you should be asking. Ask yourself "What is the thing I would most like to be doing now?" and "What could I be doing now that would have the most impact?"

These are both kind of vague questions, but that's because you have to answer them for yourself. If the answers are the same, then you have a goal to work for that will probably motivate you quite well. If the answers are different, then you can decide based on whatever feels best for you.

You might also consider posting in some of the FIRE-related forums, particularly on Reddit. There are some good resources there, not the least of which is the community of people who can tell you how they approached questions like this in their lives.