I love Crusader Kings, but I think it will be a long time before we see Crusader Kings 3. Crusader Kings 2 still looks fine (HoI4 looks better with more stuff going on on the map, but it's not a huge jump) and has years of extra development sunk into it. Crusader Kings 3 would have to start with significant less features and take quite some time to catch up with the worthwhile features from DLCs.
To justify Crusader Kings 3 it would have to have some bigger changes in the concept or a jump ahead in the supported technology. Who knows, a VR Crusader Kings where you sit in your virtual throne room with a map of Europe before you might not be so many years in the future.
So I'd rather see Paradox bring Victoria with part 3 to the graphics and handling level of their other grand strategy games and then expand the series with a fifth game: Cold War or Rome/antiquity.
If we're getting a VR Crusader Kings then I would hope that we're not going to be trapped at a map but able to interact with courtiers face to face and so on.