Personal unions and interactions with the clergy need to become a core mechanic

The single greatest unifier in Europe was religion. Conflicts between Catholic powers were usually constrained by shared legitimacy structures, dynastic claims, and diplomatic norms, which meant wars were more often about leverage and succession rather than outright political annihilation. Wars of succession in particular could become extremely destabilizing because they escalated disputes over legitimacy rather than simple territorial gain.

The Reformation broke that shared framework and reintroduced a much sharper confessional “us vs them” state logic. In that sense, the Reformation didn’t just add religious tension, it removed a key constraint on how far wars between European states could escalate.

Religious authority also functioned as a real political constraint on rulers, often limiting or delegitimizing certain forms of expansion or aggression in ways that are not well represented in EU-style systems.

In some ways, the Papacy had more institutional influence over 1300s Europe than 800s Europe (CK3 vs. EU5), despite Paradox mechanics implying the opposite

Personally, I think personal unions and interactions with the clergy need to become the focus moving forward. Although Asia is an even worse mess at the moment, but that's a topic for another day