I've definitely gone through the same, thinking of an ex-GF. I think more than anything, it was a way for me to accept my new life, while still holding on to something familiar.
Looking back, I have also realized that there are a LOT of similarities between her and my STBX. It appears that self-centred, cruel princess has been my "type".
So now that this ex is on your mind, enjoy the fantasies, but also do some good hard analysis on how well your picker is working.