I pay my respects to your fine research and concept work, Kingdom of Hungary is a legit and worthy civ despite half of launch civs being from Europe. Nice job there with the immigration mechanic.
After an investigation into the subject, it was determined that Hungary was indeed of the strongest and most influential medieval Euro nations. Therefore it was then included in the ‘Fortress of Europe’ DLC (name in its honor) as a candidate DLC in the first realistic, context-based AoE4 DLC poll, welcome to read and vote:
In fact, after the ‘Rise and Fall’ civ analysis, I would personally have included Hungary or Bulgaria as launch civs instead of English, if I really wanted 4 European civs.
The reasons are straightforward, the famous Black Army was more unique and much stronger, more well-trained and professional than the English during 80-90% of the AoE4 timespan, and Hungary was indeed the true Fortress of Europe, as well as having many more interactions with world civs than English. Hungary - all alone - defeated the Mongols in 1285, in fact it utterly destroyed the invading Mongol army, saving Europe big time (but a century later, at the great Battle of Nicopolis 1396 ‘The Last Crusade’, an all-European army of 20 nations including Hungary would be annihilated by the Ottoman Empire).
Take a look at the ‘Rise and Fall’ civ analysis: