There is one really important patristic text that isn’t online. I refer to the massive compendium of heresies, the Panarion of Epiphanius of Salamis. An English translation was made by Frank Williams, and published by Brill. Massively expensive, I cracked and bought a copy some years ago. It is the main source for the Ebionites and Nazorean heretics, for instance.
It seems that a new edition of the translation has appeared. Kevin Edgecomb has done a rather excellent review of it, which indicates that it is a thorough reworking. Indeed the old one was rather stilted, so it needed it. Unfortunately he still translates “heresy” as “sect”. It will be a while before I lash out for it, tho!
I found Kevin’s post quite by accident. This evening I was browsing Quasten’s Patrology vol. 3 casually, and found that an English translation existed of Epiphanius work De gemmis. The work itself is lost in Greek, but a complete version exists in Old Georgian, and was translated in 1934 by R. P. Blake. Fragments also exist in Armenian and Ethiopic.
A google search revealed that Blake’s book was on Archive.org here, much to my delight since it turns out to be a rare book. I will try running that PDF through Finereader 9 and see if we can get an OCR’d text of the English translation.