EU4's standout point is its immense variability in experience from country to country. The span in playstyle the game can elicit from playing an HRE prince to playing a steppe horde is a huge reason to continue building on the base that already exists. That quality will doubtless take time to be built up through patches and expansions in any sequel.
(I do enjoy this game, and nothing below is meant to imply I don't still appreciate it.)
On the other hand, the factor that comes to mind for me where EU4 needs improvement is in the vibrancy of the game world. I don't say that as merely a mindless Paradox critic claiming they can't make an AI, but somewhere along the line I do think EU4 lost the thread. The game has so many mechanics that the AI doesn't seem capable of capitalizing on. The number of different diplomatic interactions the player can perform is great, but ultimately they can feel like simply turning knobs on a control panel to make the game do as you say, not meaningful interactions with competing entities in the world, who can do the same to one another and to you. In the battle of unstoppable force versus immovable object, the AI usually proves to be all object, no force—and a rather movable one at that.
That is to say, in so many words, the AI mostly just sits and waits for the player to arrive before it, too, is annexed like all the AIs before. It might work halfway through its mission tree; it might not. It might tie itself into a big alliance bloc, but that doesn't mean it's coming for you, and you can break it apart another way. And then, WHAM, you have a coalition spanning half of Eurasia, because everything up until now was fine, but Vijayanagar just can't stand for your vassalizing Switzerland, sir! This isn't a critique of whether the difficulty of the game is too high or too low, or pro-blobbing versus anti-blobbing—it's just not a very realistic-feeling interaction with the rest of the nations that populate the map.
I'm late to the Paradox party, so to speak, but I get the feeling from what I know that EU4 has outgrown its skeleton somewhat. I just wonder whether building a new game from the ground up, with an honest understanding of how many features the devs intend to stuff into it, could result in a product that ultimately is more engaging to play.
