I was considering Wallachia, esp. with the excellent Vlad Tepes LH available, but it eventually lost to the 31-civ and population density limits (just like the Flemish, Persians or Teutonic Order & Kingdom of Jerusalem, which were postponed to a scenario). The main arguments were that for the most part of Middle Ages, Romanian lands were either under control of Hungary, Bulgaria (and that's the usual settling pattern on the map) etc. or seperated into 3 states, as you mentioned, Moldavia, Wallachia and Transylvania. Then, the map is rather densely populated in the Balkans. I'd definitely include Wallachia in Late Middle Ages when it was most prominent, but otherwise, I had to make 31 choices for the basic 700-1550 game