Part of why they're running down DLCs for CK2 now is because they're basically 'running out of room' in the engine/architecture of CK2. It just wasn't designed with all the stuff and mechanics and modifications they've added over the years in mind.
Hopefully they learn from that and any CK3 they make is created with even more expandability in mind, and don't underestimate it. And just general flexibility in the code, both for dev coding and modder coding.
I mean, just for a start, using the Pope as a target for 'must happen' events and stuff means they can never get rid of the pope, he ALWAYS must exist, and also precludes enabling the player to play as the pope, even if they got around the whole 'dynasty' thing (I still support ideological 'dynasties' for making theocracies playable, personally, while still keeping in the spirit of CK2). Even if it makes zero sense for the pope to be around and still calling crusades (no catholics left, no catholic bishops, rome is pagan, the schism is mended, the fraticelli have taken over, etc etc etc) he *must* exist or several key mechanics in CK2 just break. I think they've been trying to migrate somethings to either non-target or to target someone else, but I think some things just aren't migrateable.
And also the way CK2 is coded makes it nearly impossible to have landless/holding-less characters playable, which I think is why Nomads are the way they are.
Not to mention the difficulties in adding China - personally I like the way they implemented it, but part of the reason they didn't just add in China in the first place is because of the trade-off was too big between having a realistic number of provinces for China and not blowing up the god-damn CPU in trying to run it. (beyond how European feudalism is too much of a stretch for the Chinese empires societal structure.
AND that also raises the point that part of why theocracies are not playable, besides the dynasty thing, is that CK2 gameplay is still fundamentally about a feudal structure of lord, vassal, and courtier. Theocracies are just not about that, and I don't think CK2's code (and gameplay) can handle a non-feudal structure for a player. Tribes are the closest to non-feudal and even then, you still have suzerain and vassal relationships. Republics are broken as much as they are because of how CK2 is so permeated by the assumption of feudal. Them adding specific governments rather than depending on capital holding helped fix a bit of it, but they're still fundamentally broken both on a gameplay level and on a buggy code level (I can't being to count how many times I've had a good republic game ruined because the game decided to revert me to feudalism due to buggy code, even with the addition of governments.)
So yeah, CK3 may not have as many mechanisms and such as CK2 at the start (that's a LOT of coding to front-end, almost 10 years worth of near constant development to package in 2-3 years for a new game?) but I should *HOPE* they at least think about all the things they added to CK2, and all the things they didn't add because of limitations, and all the things they did add but had to mutate to conform to CK2's basic assumptions, and make the code-base a LOT more flexible for future CK3 DLC and patches that do add them. Playable theocracies, inland republics, non-feudal based realms, playable non-land-holding characters (especially for republics, allowing more than 5 patricians and allowing them to migrate to different courts and republics as they historically did would benefit a lot from not requiring landed holdings, or the patrician manor) all these are things that the fundamental architecture of CK2 just doesn't allow.