I don't think late game dlc would fix the problem. . I think that the thing which would change things with players is having games run as long at first bookmark as they do at the last one... people just really don't want start dates that don't give them as much time as others
No, it would change literally everything. Right now game is static as hell- no point in playing and playing, unless You want to paint even more map in Your color. I'm not talking about adding later dates, I'm talking about state
evolving from what we know from CK2, towards the beast it is in Vicky 2. States progressing slowly would be left behind, they would experience financial problems (like PLC in XVIII century), and they would have to somehow manage with smaller armies.
Your cores never change unless they are added or subtracted by event. So if you are Korea and you conquer a random Chinese territory, the nationalism will eventually go away but it will always be a non-core territory.
That would be overkill I think, but current rate of gaining cores is definitely too fast (and too simple...).
BTW- Technically, You could mod this in- simply increase coring cost by 10000000%, so no one can ever afford buying a core.