In an earlier echo of the Millennium Bug worries of 1999, there was considerable angst in Europe as 1000 CE approached, in terms of the return of Christ, end of the earth and everything between. In reality, nothing happened of course, but what if it did?
The year 1000 CE makes a great premise for a campaign based on something actually occurring; demons walking the earth, aliens landing or whatever.
The Papacy is corrupt and chaotic, with the colourfully-named Rule of Harlots period, Aethelraed Unraed is King of England, the Vikings are very much alive and kicking, and Emma of Normandy is a teenage girl. And that's just for starters. Figure in a necromancer raising undead Roman legions and you have a wild ride!