I think Rdf has it right here.
For maximum Germanification, you're looking at a Majorian Wins scenario. You have Visigoths, Burgundians, Franks, Vandals, Suebi, Ostrogoths, all within the Empire. You can even add in the Saxons if you're considering Britannia.
I have an image of a WRE where laws and civilian life are largely Latin - the ruling class talks amoungst itself in Greek, but the military hierarchy largely uses Latin and some Germanic, whilst the common soldiery uses both Latin and German in large amounts.
At least for a few generations until a Germano-Latin emerges, presumably in Gaul, and slowly spreading from there. At this point, there are too many Germanics in the Empire to assimilate using traditional means, so the Romans will have to adapt.
I imagine that Historians would call this the "Gothic" or "Germanic" Empire, much in the same way as the East was called the Byzantine IOTL.