One thing the game is missing is the fear of losing power. Generally, going republic is the obvious choice: you can choose your leaders, the leaders are better, and the bonuses are better. In real life though, republics didn't prevail until very late, because the ruling class didn't want to lose its power. What if EU4 would implement a similar mechanic? Republics would be better, but the player couldn't do whatever he wants. For instance, he might need parliament's consent to declare war. Creating a too large army might spark protests against militarization, or a too small army might spark protests against the weakening of the nation (depending on what the public mood is). Prestige should also fall quite often due to "scandals", lost wars (more than in non-republics), even lost battles. Perhaps restrict player's ability to just change investment from eg. 100% land to 100% naval. In general, the player should want to balance more bonuses vs. losing power.
