What's wrong with current West European sprites? I think those look quite nice.
And let's not forget that mechanics of the game are modeled after Western Europe. Muslim and Byzantine DLCs enhanced areas which didn't fit to that model. I think that most fitting Western Europe expansions would deal with merchant republics and Catholic Church, because feudal monarchies are already well covered. In my opinion Eastern Europe needs more attention than the Western Europe, because feudal mechanics don't really portray that well the internal struggles of Slavic ruling dynasties and their inheritance system and then there's plenty of Pagans who could have some attention.
Well, I'd think there's plenty of things, that could be done for the celtic world as well
But that being said I'd still like for more love to eastern europe. I never play over there since it just dosn't feel right..