Given a scenario where Justinian (and his heirs) is not retaking the lands of the former WRE, is it possible that at some points the Germanic kingdoms formed in the former WRE (Ostrogothic K., Visigothic K., Francia, Vandalia etc.) might federate in some way in order to better protect themselves against powerful common enemies like Arabs, Vikings, the Byzantines in some context...?
At first these different Germanic entities rivaled between them but it is possible that at some point all of them might have established more or less stable common borders and also converged into a same Christian faith (probably Catholic).
Probably the expansion of Islam would have been not the same with Ostrogoths and Vandals in good shape, might be the Muslim armies would have been stopped at Libya. It is also possible that Avars, Hungarians etc. might have been halted and kept out of Central Europe.