In 2005 FDX was going to replace the 727-100/200 with 737-400s, and they swore the 757 would never work. They couldn't find enough 737-400s that they liked. We now have around 60 of the 87 757s that we 'ordered'. Once we got the 757, they claimed we would never have the 767, as it wouldn't work at FDX, we have 50 on order currently.
The 737-400 seems to be doing well as a freighter. With the 737 MAX coming within the next decade, there will be a lot of middle aged 737-800/900s in service looking for new careers. I would imagine they would be fine freighters, and perhaps UPS or FDX would take a look. Whether or not they will buy them is anyone's guess.
I have heard 'rumors' that we have looked at 737 again, but they are just rumors!