The only Emperor's in the game should be the Byzantine one and possibly, an added one for the Holy roman one...both derived from the Romans...remember, the Pope used the justification that there wasn't any Emperor in Constantinople when he crowned our favourite frank...
Numerous other rules have called themselves emperor. Bulgarian (913-1018 and 1186-1393) and Serb rulers (1346-1371) for instance...but simply handing out that title to all who have at one point called themselves emperor would simply cause the title to lose it's meaning. I say stick with it as the title for inheritor(s) of the Roman tradition.