Which of these European civilizations you would like to see included in civilization?
Belgium
Bulgaria
Czechoslovakia
Grand Duchy of Lithuania
Hungary
Kingdom of Bohemia
Kingdom of Croatia
Romania
Switzerland
Ukraine
Yugoslavia
I've eliminated the ones that could be represented by a current Civ.
Then, we eliminate the overlap in suggestions:
Belgium
Bohemia
Bulgaria
Cossack Hetmanate (rather than Ukraine)
Grand Duchy of Lithuania
Hungary
Romania
Serbia (rather than Yugoslavia)
Switzerland
Now first, it's important to look at the practicality of having each civ, assuming that every civ has to have existed for 100 years or more. (most did, except the Huns and Mongols I think all of them did.)
Belgium: Works well in a Scramble for Africa situation but how would you represent Belgium in a normal game of Civ? Remember that Belgium spent a lot of time in history not being Belgium. You could have an economic powerhouse Belgium, which would probably fit them best, but then they would be a highly situational civ.
Bohemia: A bit easier to work with than Belgium, could be considered a civ which is fairly religion based, but could require a rework of the Piety Tree.
Bulgaria: Would be military based, but there are a lot of those already.
Cossacks: Once again military based and would require a rework of Russia.
Lithuania: This could actually be a really interesting one but once again, how would you properly represent the structure of the nation? Wouldn't it be better to change Poland around a bit?
Hungary: As above, wouldn't it be better to change Austria around a bit?
Romania: It's a bit of a modern civ, but so is America. There is probably a way of incorporating all of the Principalities into it a little bit, but then again, all the Dracula jokes would be too much to handle.
Serbia: I'd rather have a Serbia than a Yugoslavia, although there may not be a way around it. I also have no idea how Serbia would work. Maybe Belgrade should remain a city-state.
Switzerland: Switzerland is... ineresting. How would a Swiss Civ work? Gifting units to Civs in return for a peace treaty?
Some of the civs are not bad ideas at all, it's just that there's a certain balance to be maintained amongst civs, and some wouldn't really work as civs anyway.