Not to continue doubling down on AHC scenarios, but what about a Western Roman rump state surviving past the fall of Rome? IOTL these states existed in several...unfortunate...locations, including in France and in Dalmatia. What about one in Britain? Bonus points if it can persist and not merely exist, creating essentially a Western Byzantium (and if Romanized natives eventually take the Imperial title, that would be good too)
(The reason I ask, of course, is that the idea of a Britannio-Roman Emperor ruling a Roman successor state of Britain is...well...pretty cool.)