This specific post was posted in another thread, but it was discussed here too, so I will reply here.
I do not think that a Victoria 3 should have EU4 diplomacy. With that, we would loose the whole system with spheres of influence and puppets that Victoria 2 has, which would be bad. Adding vassals and royal marriages, while not crazy, would not be the most relevant to the timeframe. Guarantees or protectorates though, might be good for a Victoria 2 game, as well as a functionality which would permit the player to constantly keep improving relations with a country. Supporting rebels is another interesting feature that would have it's place in a Victoria sequel.
With this said, my personal opinion is not that a Victoria 3 diplomacy should be based upon the one in EU4, but that it rather should be strengthened by it while keeping the Victoria base (alliances, spheres of influence for GP, puppets, crisies), with some elements used, and others not.