I don't care for the word "legacy" (the connotations of the word bothers me), but we're definitely in the "any more we earn in FIRE will be donated to charity" (along with any remaining stache).
IMO, most parents wanting a bigger legacy for children would be better served FIREing and spending the time with their children, but donations to charity... well, that's right in line with what's important to me. Hard to argue against giving food to the starving, or educating the illiterate, or preventing someone from getting malaria, or getting people clean drinking water, or whatever your particular cause(s) is (are).
Of course, the question becomes... why OMY? Why not two more years? Or 5, or 10, or work until you're 75, and donate it all? We all draw our own lines, and hopefully everyone will do what they can.
I think your OMY is a great place to start, Financial Ascensionist! Hope you keep making a difference in FIRE and beyond. :)