It's really hard. The Germanic tribes pretty much wanted to inherit the Roman system with themselves on the top. They were bound to be assimilated. Only in Britannia did Latin die out but it never had much of a presence outside the cities in the southeast which were the first to be conquered by the Angles, Saxons, Jutes, and Frisians.
All quite true. Here is more for anyone:
http://www.thenagain.info/webchron/westeurope/AngloSaxon.html
Specifically: "... the Caledonians began to attack them. Britons requested that the roman government provide military aid, but it never arrived. Faced with no other options, the Britons turned to the Anglos and Saxons of Germany and the Jutes of Denmark for assistance. After the defeat of the Caledonians, the Jutes were given the Isle of Thanet. Jutes began spreading into Kent, and as a result, were defeated by the Britons who lost Kent once again to the Jutes around AD 488."
No Caledonians nearby for most Western Europe areas, so the only power that makes much sense for this scenario is one of the many Asian horse armies. Problem is not only are "The Germanic peoples of the fifth century were very destructive and barbaric in nature. They preferred to kill their enemies although they did keep some as slaves" (which means the Asians would have to be far more terrible to even consider Germanics as potential allies) but also Germanic tribes are situated in a keystone area between the Asians and Western Europe.
Which logically follows that Germans are going to be scattered and made to suffer, too. Or maybe part go into exile (the potential allies) and all the rest go into the army of these Asians (which did happen in the case of Attilla), and then have the situation mutate in some way so most or enough of the Germans come on top.
As others have said, a difficult situation with a great many variables. Too dicey for my blood. Good luck in all.