Before Möcker comes in and tells us new factions are still very much, as we say in my country, 'future music': I'm aware of this!
Having said that, I'm a bit torn about an Imperial Roman faction. If I'm not mistaken, the Holy Roman Empire was quite diverse. How would all these different elements (mainland Saxon, Frisian, Thuringian, etc.) be reflected within a single faction? Moreover, this game is not so much about large battles between armies as it is about small battles between warbands, and it seems to me most of the smaller battles of the time would not have been fought by the Imperial Roman army, but by local forces. More importantly perhaps, this game is centered around on Vikings foremost, it's about fights between Vikings and other poor sods. In practice, when Vikings were fighting 'the Holy Roman Empire', they were fighting mainland Saxons and Frisians specifically. Lumping the mainland Saxons into a Imperial Roman faction would be akin to lumping the Normans into a Royal French faction. For as the Duke of Saxony was a vassal to the Roman Emperor, the Duke of Normandy was a vassal to the French King.
But, on the other hand, the Viking and Gaelic factions are also generalisations, where diverse groups are also lumped together in a common faction...
Still, I would prefer a separate mainland Saxon faction.
Oh, and as for the relationship between mainland Saxons and Wends, what about battles like the Battle of Recknitz