Well, it's admittedly not a very much vulgarized regional history (bit helpful that I had access to Le Bohec, Modéran and Laroui to check firstThank you for clearing things up about the rather complex political situation in Africa. In my attempt to draft a quickshot out of my head, how Africa could become independent, I obviously messed a bit around.
Oh I agree with you on this, but it's more of a last drop than a pivotal point itself : a Roman Empire that would still count on a not too damaged Italy and some provinces (let's say, without exclusive, part of Spain or southern Gaul) could have enough ressources to hold on on its own.If we look to Peter Heathers theory about the Fall of Rome, the invasion of Africa by the Vandals was the point of no return. Even if a lot of historians diesagree with his so called shock-theory, almost all agree to this point of no return.
Well Franks and Saxons raided western regions up to Hispania, Herulii and Goths up to Greece, so I wouldn't rule out a Mediterranean Barbarian presence with a united Rome collapsing.It is highly unlikely that a german tribe of the 3rd century migrates as far south as the Vandals did in the 5th century.
That said, I think that Barbarians (which may be more Germans and Sarmatians, rather than just Germans there, even if much of the latter were thoroughly sarmatized) would be that of a political force they were in the Vth IOTL without being as structured they were.
A more gradual political takeover, not unlike how Turks took precedence in Xth/XIth century, may be more likely in a first time.
You'd argue that it's what happened IOTL, but it's a bit different : with the IIIrd and IVth revival, Barbarian groups were even more importantly romanized and structured along Roman political lines, "fitting" more the late Imperial model and more quickly absorbing Roman elites and being on top of Romano-German kingdoms.
ITTL, I'd expect "dukes" or emperors wannabe to use them as Aetius used Huns IOTL : a convenient taskforce; that would likely take more power with time.
It's how you may end to see Barbarians in Africa : a barbarian military force being considered as more trustworthy than locals with more immediate and clear political agenda; and eventually taking on (partially or completely) "ducal" entities.
Would that fit your propositions for the OP?