There's been excessive feature bloat for a while now (shiny new buttons, modifiers stacked atop modifiers, etc, etc).
There's been a lot of new surface stuff added that hasn't really innovated on the Grand Strategy genre or added more depth to the game. There's definitely some mods that iterated on the base concepts (I generally dislike mods, but Meiou and Taxes made me really enjoy playing tall) and added some meaningful complexity to decision-making: a good example is handling estates in a way where if they get too powerful it will take effort to rein them in (i.e strong bonuses from them, but privileges added to nobility will add stacking penalties income or prestige gain etc), but giving them some power base will make them improve the provinces they're present in, etc.
Overall, EU4 is not in a complete state; some of my favourite parts of the map have no meaningful missions/formables; the map could still use some polishes; colonizing the Americas is kind of worse off than it used to be, and so on. That said, it's complete enough, and it's old enough, that I'd prefer ending development on it and starting on EU5.