Maybe have Constantine somehow remain in Brittania, rather than marching on Rome.
Alright, in 307, as Italy was in anarchy and civil war again, Constantine tried to remain neutral. Maybe kill Maximian here, and have the Empire descend into more civil war. Eventually, the only figure with any true power is Constantine, Caesar of Brittania and Gaul. He or his successor lets the Germanic tribes invade the rest of the Empire, provided they spare his territory, and Iberia and Italy are devastated, furthering Eburacum's status as the only stable center of power in the West. Eventually, the Romans of the West turn on the barbarians, and expand into Germannia.
Sound plausible to anyone?