So in my archaeological travels of the nether (while searching for the long lost "Spleef Club" training grounds - read here) I have found the location of 3 more Nether Fortresses aside from the one nearish to spawn. They do appear to spawn on the same axis so if you look anywhere around the X:550-650 area you should find some. I've included below in the following post pictures with coordinates. They were all massive so there is way more than enough netherbrick for those who want some. 2 of them also had easily accessible blaze spawners.