The territory of the Bulgarian Empire is de jure Byzantine, as it should be. Indeed there are Northern ahistorical "empires", but these are all situated in areas where no larger neighbouring empire has a strong imperial claim to their territory.
For all intents and purposes, Norway was more an Empire than Serbia/Bulgaria in the CK2 time period, as they were far more expansionist and none of their territory was claimed by the HRE.
I support the ahistorical empires to a point (Scandinavia, Britannia, Spain and Rus are more legitimate than Francia, which should technically be part of the HRE, perhaps with a potential titular "Francia" to serve as a renamed version of it), but there comes a limit where we have to accept the true nature of each country.