Orly? When did the Norwegian sovereigns were internationally recognised as a Caesars of the Norsmen or whatnot? You are mistaking the modern definition of an empire and the historical titulature used during the game's time frame. While the kingdom tier makes sense for Bulgaria and Serbia (and BTW having king vassals for Byz does not), there is a significant enough distinction between the titles in a time where that mattered. While I doubt it would be added as a stand alone DLC, it would have made sense to be a part of LoR.
As a matter of fact neither of those does make any sense. The unification of most of those regions came to be as personal unions and maintaining those would be the point of unification, while the Russian claim of ceasarship is to much out of scope of the game. The only emperor running around those places during CKII was in Spain and that should also be addressed by a separate mechanic. Titles, however empty they may seem to be to you, were of great relevance in the era, something I consider would have been a nice boon to the game.
The roman word Imperium loosely meant authority/dominance, and so could be applied within reason to any European dominant power (where it holds a majority of military strength).
I used Norway as an example of an expansionist power which satisfied the modern definition of an Empire.
Another weight to my point was the fact that places which would have called themselves kingdoms are regularly called duchies in the game, which is completely unrealistic in Northern Europe.
The long and short of it is that in CK2's simplified system, kingdom makes sense (giving them "empire" status would grant them inaccurate prestige bonuses and too much sway in Europe). If Paradox decide to overhaul the system so organisational structure is no longer linked to nominal title (so a king can rule an empire, an "emperor" can rule a kingdom and a "king" can rule a duchy) then I'm all for the change. Indeed Bulgaria did call itself an Empire in the Middle Ages, and it's inaccurate to see a king of Bulgaria where an emperor should be the actual title. Equally Cornwall considered itself to be de jure Kingdom of Cornwall, and it was more or less recognised as such by Danes, Bretons , Welsh and Irish.
The leaders of Irish "duchies" called themselves Kings, as did those in Wales.
There is a lot wrong with titles in the game, and to change Bulgaria they'll have to change them all, which I just don't think they'll do.
CK2+ does a good job at adding in these titular kingdoms however, so it might be worth giving that a look (and putting your case forward to the author if it doesn't have titular empire of Bulgaria.
. There is no Italy, but Middle Francia (Lotharingia) gobbled up by the eastern Franks. In any case, I think one imperial title descending from Charlemagne should be enough. Currently every region of W Europe has an empire, which makes the HRE not really special enough, I'd rather see the player and AI trying to get the title from the HRE, a bit of deficit goods. 
