I think that EU4 Ruler/Royal family system is to simple and should be expanded on few ways. For inspiration I use your ruler system from Imperator Rome
1. that every King can have more than one children not just one heir and one consort. Diplomatic Marriage been important thing to make allies in Europe in period which EU4 represent
2. That we can decide which Prince/Princess we can offer for marriage to other Country Royals
3. that get its pictures and more complex personality traits on one side and diplomatic/combat skills on another
4. that we have family trees
1. that every King can have more than one children not just one heir and one consort. Diplomatic Marriage been important thing to make allies in Europe in period which EU4 represent
2. That we can decide which Prince/Princess we can offer for marriage to other Country Royals
3. that get its pictures and more complex personality traits on one side and diplomatic/combat skills on another
4. that we have family trees