Wow! Even as a relative newcomer, I sucked in a breath with that one.
I do want to check out I:R, simply because I have a feeling I'm trying to constantly do the Roman Empire thing, but in the Middle Ages.
I have a feeling its going to end up like Victoria 2, with a cult following of people who are really hardcore and passionate about it (including me). The narrative justification for whatever you're doing is very thin in I:R and it doesn't have a strong sense of immersion in the way some other games do. As a strategy game though, I think it is wonderful and if you can find a personal challenge that you enjoy you can easily get into doing it over and over again.
Ironically, I think it can be a frustrating game to play very wide in because you start spending a lot more time putting out fires (figuratively) which is a bit more reactive than I like to play. For me, the most fun achievements are forming small but powerful nations and trying to push them as hard as possible with limited territory and resources.
Basically, imposing limitations and then seeing how far you can go within those limitations is just extremely satisfying when you have the right mechanics to do it. That said, going full roman empire world conquest mode would be complex and challenging enough in I:R that a certain type of person would definitely be really into it.
What can be improved: add plagues. New governments, basic economic mechanics.
Plagues, sure, those were pretty fun and flavourful if a bit insubstantial.
Basic economic mechanics. Sure, the ones in CK2 did kind of feel like cheating sometimes. I would like to see the CK3 version be less scripted and something everyone can interact with rather than just free money for some regions.
New governments. Please, no.
Thematically, horse lords is my favourite CK2 DLC. I love nomads, it's a part of history I'm really interested in and would love to see represented properly. The problem is that nomads introducing a whole new set of mechanics that worked completely differently to everyone else was a mess. It never worked from a balance perspective and resulted in an incredibly easy and boring playstyle which completely glossed over the core gameplay that made CK2 enjoyable. To counter this, the limitations imposed on nomads didn't really solve the problem and were just annoying.
I have given quite a lot of thought to how I'd personally like to see nomads done in CK2 when they get round to it, and I might make a suggestion at some point once I've hammered out those ideas, but one thing I'm absolutely certain about is that nomads should use the existing tribal government mechanics (with variant flavour). Those mechanics are fine, they work.
The same is kind of true of republics. I know republics had fans, and I know some people enjoyed playing them, but they were kind of a mess.
CK3 is not a challenging game for veterans, is not a balanced game and is not a cohesive game. To an extent, I would even say it's not a strategy game, since I only have to click my win-button of choice and the game will adjust accordingly, handing me my requested victory.
So, I see this a lot and I don't get it.
I have well over a thousand hours in CK2, enough that if I don't qualify as a veteran I don't know who does. CK2 is a game I find incredibly easy. The only fun I have ever had with it after the first few hundred hours is based on intentionally limiting myself and roleplaying. It is not difficult. At best, it has some basic anti-blobbing mechanics which CK3 doesn't have, but they're easy to overcome.
I don't get what people are doing in CK3 that is making it easy. Like, there are clearly unbalanced features, but nothing I would ever describe as a win button (other than maxing court amenities which is clearly not WAD). Overall things seem so much better. Even just the fact that it is so hard to get out of partition adds a huge amount of difficulty which was utterly absent from CK2 past the extreme early game on some starts. The stuff that does actually seem imbalanced mostly seems like a choice players are making, rather than something baked into the mechanics themselves.
I will happily concede that maybe I'm not good at CK3, but I seem to be good at CK2 so clearly it is possible to be a "veteran" and still find CK3 more challenging than CK2.