I think anywhere in the southwest or west in general would be workable, although I prefer non-snow places. Maybe I'd put up with some snow if I was FIRE'd because at least travel would always be on my own terms, not driven by a work need to get things done at a certain time. Areas I might consider would be:
(All are general areas, not necessarily within city limits of the named location)
Fort Worth, TX
San Antonio, TX
Phoenix, AZ
Prescott, AZ
San Luis Obispo, CA
Tillamook, OR
Provo, UT
Colorado Springs, CO
It's hard to say exactly which places I'd absolutely live year-round versus seasonal living, as I'm not sure what my life will look like 10-15 years from now when I'm likely to be boarding the FIRE train. I think places like Phoenix and Fort Worth are just so easy to find a nice, affordable place while being surrounded by city services and close to an international airport that it's easy to say I'd live there year-round. Further down the list, I could maybe picture living somewhere in the San Luis Obispo area, as I've found rentals there which are pretty price competitive as long as you're willing to live small or be a roommate.
For the last 3, I love those places but have also only gone in the warmer months. I don't like snow and I certainly don't like the prospect of riding my bike to the grocery store in the snow. I have even more trouble picturing myself at 55-65 years old enjoying riding my bike through the snow. However as a seasonal deal, I could picture myself doing a cheap rental in a cool weather place from April-September, then back to the a warm place near family from October-March. That way I kind of get both things I enjoy: mild weather year-round, but also close to family at least for half the year and all the major holidays. SLO, CA would give me nice weather year-round, but none of my family is in California.