One of the texts that is not online and really should be is the legal compedium assembled in the reign of Theodosius II in 450 AD and known as the Theodosian code or Codex Theodosianus.
The work was compiled from earlier collections of imperial edicts, or rescripts as they were known. These took the form of a letter from the emperor to some official, usually a proconsul or prefect. The compilation provided a systematic list of things proscribed or permitted.
The work was translated by a certain Clyde Pharr back in 1954 for Princeton University Press. That means that it could be out of copyright in the US; unfortunately it is not.
The most interesting portion of the code is the last book. This consists of the rescripts on religious matters issued by Constantine and his successors, which progressively made Christianity a privileged religion, then the state religion, and then prohibited other religions aside from Judaism. The tone of these rescripts is often violent, as is often the case with the edicts of later emperors. Pharr’s introduction points out rescripts which indicate the powerlessness of these emperors, and their repeated and futile attempts by ever heavier penalties to get their will enacted by the imperial bureaucracy.
Such interesting material is always likely to find its way online in unauthorised form. Today I found a site with a substantial chunk of that book 15 here. I’m not sure whether it is complete, tho.