on the emperor thing:
In the Latin languages the title emperor come form the latin word Imperator. In French this gives Empereur. The English language is influenced a lot by French (1066 William the conqueror) so it became Emperor.
In the other Germanic languages, the Emperor title comes form the name of the first Imperator Octavianus Caesar (= Emperor Augustus) who took over the power from Julius Caesar. The emperors of Rome were named Caesar. In German this gives Kaiser, in Dutch Keizer (almost same prononciation). The Latin prononciation of Caesar sounds more like Kaizar anyway. Remember that all Germanic Empires of the West (Franks, Wisigoths, Allamanic,...) saw themselfs as vassals of Rome (later Byzatium). I always though Tzar was the same as Russia saw itself as the 3rd Rome (1st being, well... Rome itself, 2nd being Byzantium) so the leaders of Russia took the title that was rightfully theres.
Maybe it would be wise to use the generic title empire or emperor, but it would be nice to add some flavour and use the other titles too. (If Belgium would have been a civ, what would it be? The generic Kingdom of Belgium or the flavoured Koninkrijk België - Royaume de Belgique - Königreich Belgien?)
I also think that using the civic-bound names for empires could pose a lot of problemes. Example: Spain has a Hereditary Rule and Universal Suffrage. So Kingdom of Spain or Republic of Spain?