I am currently in a 15 year relationship with my high school girlfriend... 35 yrs. in between....

No one-size-fits-all here...

Just remember, there was a reason he became an ex. Speaking from personal experience, here.

There's one flame I'd like to rekindle, but I don't see it happening. Unless, of course, he decides to leave his wife.

I've rekindled several old flames, some more than once. I've found all of those experiences to be positive.

I've seen it work for some. But though I've remained friends with some exes (and their new women), I've not ever rekindled with an ex.

There was one however, that I would have given it a shot with, if he'd been willing ... long ago.

It's like I said to someone else here, if there were issues with the person and/or your relationship drynamic (ie bad conflict resolution) then those problems will still be there a 2nd or 3rd time around... but if the relationship was ideal and fell apart due to timing or different life trajectories (ie pursuing careers in different regions or one wanting to start a family and the other not wanting kids) then the relationship can be rekindled without problems.

Everyone is different. Once I walk away, I'm gone for good. Relationships end for a reason. Unless both people have changed considerably, the same issues will reoccur.

there exes for a reason

Remember why you broke up the first time.

It would entirely depend on where both people are with themselves and each other. Both would have had to have gotten past whatever problems caused them to break up, and to be able to work through together any challenges that would come up in the present. Or if they more or less drifted apart, they still need to make sure they're ready to come back together and that they're doing so for the right reasons.

It depends on whether one or both sparks have changed, or not. At the minimum you may enjoy reliving a bit of history. But, more than likely, it will not work out forever.

My inclination is no. However, it depends a lot on what kind of relationship it was before, why it ended or the two drifted apart, and probably most importantly if trust was broken. It can be complicated, and those pesky feelings screw things up.

I'm up for anything.

No surprises.