For historical accuracy, and also for gameplay to some extent, eu4 needs more types of subjects. Or tributaries need a major overhaul. Currently, there are situations that demand a different system. The Ottomans got mentioned here already. A different situation I really dislike, is Russia: Moscovy starts out much much much stronger than Novgorod thanks to the vassal swarm, which gets integrated very quickly. Especially all the extra manpower and capital forts turn this into a nightmare for Novgorod. Additionally, several of the "vassals" got conquered by Muscovy irl. I’m also not sure why Pskov is a vassal. A different approach to subject relations is really needed here as well.
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