I do like the idea of another faction during the Third Century crisis, but I'm not sure where they could come about. Perhaps the first rebellion in Africa when Maximum Thrax was Emperor offers to surrender in exchange for political devolution, and they'll surrender the Senators who backed them (and their false Emperor). Throw in a great orator to convince Thrax - and have an African Republic that elects African senators, but is still sub-ordinate to the Emperor.
I don't think it is too far fetched. If the Republic has no military authority, but only judicial/civilian authority - then the African-Romans get their problem solved, Thrax gets a new support base, and undermines the Senators in Rome who he wasn't working well with anyway.
It also becomes a useful weapon against any rebellious would-be-Emperors. Giving local political power to the people in an attempt to reduce corruption, with an example of doing so, would undermine any usurper - and perhaps replaced the Third Century Crisis with something less disasterous. It may well end up being rolled out across the Empire, creating an Imperial Union of Roman Republics - not quite the old school republic, but still - Republican.
I don't think it is too far fetched. If the Republic has no military authority, but only judicial/civilian authority - then the African-Romans get their problem solved, Thrax gets a new support base, and undermines the Senators in Rome who he wasn't working well with anyway.
It also becomes a useful weapon against any rebellious would-be-Emperors. Giving local political power to the people in an attempt to reduce corruption, with an example of doing so, would undermine any usurper - and perhaps replaced the Third Century Crisis with something less disasterous. It may well end up being rolled out across the Empire, creating an Imperial Union of Roman Republics - not quite the old school republic, but still - Republican.