One problem I have with great diplomats is that they are dependant on luck. You build an embassy in a citystate and venice decides to buy it, now you've wasted your diplomat. Also the whole idea that there is only room for so many embassies in the game that lategame great diplomats are a lot less valueable, you either rush for them or you wont be able to get any extra votes. Sure you'll still be able to stop others from getting votes from embassies by capturing the citystates, but I'm not really sure that's the best solution ever. One simple solution would be to replace the embassy option with just consuming the diplomat in your capital for a permanent extra vote, but I'm not one hundred percent sure that would be a good solution either. Votes probably shouldn't be unlimited and so on.