I personally think that EU4 should start following WoW's DLC policy to some extent. Modified of course, since the genres are vastly different, and the fact that not having the newest DLC doesn't exclude you from accessing the new content. Maybe you'd have to pay for the last three major DLCs and all of the minor packs (like Purple Phoenix, Muslim Ships, New Models,DLC content packs etc.), but everything besides that is free for everyone. When a new DLC comes out, then the one that is now the fourth newest one, becomes free. To increase incentive to buy new DLC right when it comes out, instead of waiting for it to be free, throw in the content pack associated with DLC for free, and double its price when sold separately.

I think this would enable Paradox to make mechanics from different DLCs affect each other, because you wouldn't have to account for someone not having a DLC. Besides, that would make it easier to get into EU4 for new players, because at this moment you absolutely have to buy Common Sense with it, and Art of War and Rights of Man are strongly recommended.