Honest take here, as a player I like the idea of having a game such as EU4 with very high skill ceiling, that requires real effort to master but at the same time in an ideal world it must also have a very accessible and easy-to-comprehend tutorial around the basics.
My most favorite feature for EU4 is probably missions (and flavor in general, such as events and what not) because that's what I have spent my time the most as a modder and as a feature it only reinforced my love for the game. That's not to say that the system cannot be improved, for example I sometimes feel bad having missions being the primary factor of narration / playthrough. I feel like MTs sometimes dictate too much of the pace of the game. They sure are awesome to play with, and clicking the button after completing a mission feels great, I can even say that I love love love reading event rewards, descriptions buuuut there comes a point where I would love to see some more weight being shifted to other gameplay/narrative aspects such as reforms, estate privileges, events etc. And I feel like this is one of the contributing factors to Lions' success, the addition of new reforms etc