The next generation of Paradox games would need some revolutionary core mechanic to justify moving to a new game and abandoning the multiple years of developing on the old engine and game.
One such mechanic may be full population modelling, with the entire population modelled as individual units (maybe 1k people=1 unit), with characteristics like age, culture, religion, wealth and allegiance.
This mechanic would be able to unify many disparate existing mechanics, like coring, province based religion/culture, manpower (now realistically based on population size instead of modifiers), rebellions (no more % based rebellions, also more realistic numbers), development/devestation (now based on population, terrain and perhaps the wealth/education of the pops)
The representatives of these pops would be jockeying for political power, and this would then replace estates, government types (making it more complex than clicking buttons), and changing peacetime gameplay entirely.
Populations would naturally migrate from province to province because of war, disease, economic attraction, overpopulation etc, this would also largely replace the existing colonialism mechanics, and again make peacetime gameplay far more complex.
This system would obviously be extremely complex, so even though Vic 2 and Stellaris have rough approximations of population, Paradox would be making a leap into the dark. So Vic 3 would probably be the experimental ground for this. If it works, it'll likely be adopted for CK3, EU5 and stellaris 2.