This is a good idea, but I think that in practice it would seriously unbalance the game. I can just imagine, as the good old OE, serially force-vassallising every nation to my east, right up to and including Japan, and then serially diplo-annexing them. No revolt risk. Massively increased cores. Genghis Khan would tremble at my might. Or consider the BWB, rampaging across Europe. Not even Charles V would be able to come close to My Imperial Majesty.