I personally don't see problem if EU IV become more similar to CKII in this game mechanic, in EU game period diplomatic marriages and other things related to that still be important part of politics in Europe and other countries. But I suggest much wider changes to ruler mechanic and its skills system and other things.
These games still will have big differences in other main game mechanic like ruler/vassal system, religion and many others, for me this is right time (when Paradox/EU team is bring back its focus on Europe) for refresh/rework old ruler/RF/PU system.This part of game isn't changed from previous EU games too much and definitely need refresh.
Also another pros for rework is that now Paradox get XP how can to made it (because it learn while work on Imperator Rome) and can use this XP to refresh on one way these mechanic in EU IV (I don't mean that it made entire copy/paste of Imperator Rome system but that it be inspiration for changes)
These games still will have big differences in other main game mechanic like ruler/vassal system, religion and many others, for me this is right time (when Paradox/EU team is bring back its focus on Europe) for refresh/rework old ruler/RF/PU system.This part of game isn't changed from previous EU games too much and definitely need refresh.
Also another pros for rework is that now Paradox get XP how can to made it (because it learn while work on Imperator Rome) and can use this XP to refresh on one way these mechanic in EU IV (I don't mean that it made entire copy/paste of Imperator Rome system but that it be inspiration for changes)