To say it short: Imperator is a game design failure, and it's the main reason why it feels empty. New Paradox games don't need to be empty.
Imperator was designed with EU4 vanilla as a base of gameplay (warfare, mana, general goals of the game, states being defined by culture and religion, random events to emulate most events), with the addition of characters. There are in fact very few features that are only found in Imperator. Fortunately it's changing.
One of the biggest problems with Imperator is that the general goals you set to yourself nor the ways to reach them are original or even interesting. It's basically a map painter, and the most efficient way to play is a snowball effect combined with troops spamming. Just like in EU4, you will convert your provinces to get more money or more troops and less trouble with micromanaging unrest. In addition you will also befriend som key characters but it doesn't involve special mechanics and it works basically like CK2's council. We're literally playing as nation states in the antiquity.
We don't know much yet about CK3 but it's obvious that Henrik F. has a vision. We're going to roleplay as characters (and dynasties) in a way that was never possible in CK2. We've already seen that characters have skill trees, we know there will be hooks to power the intrigue system. We know traits will come in play a lot more, to the point that they change our character portraits. The rest is looking like a refined version of CK2 (we don't see tons of new types of mana on the screen for instance).
I don't think there's a Paradox paradox/syndrome. I think it's a lot harder to make grand strategy games for everyone and not just a niche of people who mostly enjoying painting the map with their colours with a vague historical flavour (basically, the kind of stuff you find on the historical memes subreddit), and even better if it's a bit edgy. Gamers as a whole have higher expectations, and this is true for most if not all strategy subgenres. Simultaneously, so-called "hardcore" strategy fans don't understand what's happening to their favourite genres as their habits become inefficient in newer, more complex games. And they confuse the new non directly warfare oriented features with emptiness and casualisation.
In other words, Imperator was a game for the niche players by a game designer who never intended to target another audience (there was a big qui pro quo), while CK3 is designed by the genius who made CK2 what it is now, which is the game that basically revolutionized the whole grand strategy genre and made it available to everyone. It doesn't mean that CK3 can't and won't have issues at launch, but Imperator's issues were game design issues that they will try to address with many updates and DLCs, while CK3's issues will probably just be that it can get repetitive at some point, and DLCs will be more like old school expansions that literally expand the game.
