I have no idea what the premium will give us, but I suggest 2 things which I will be willing to pay diamonds for :
1. The ability to instant travel ( not shorten the time needed to travel, but for example if you go to global map and want to move to province which is 4 provinces away, you dont need to every 10 minute click on the arrow but can just once click on the province and be there in 40 minutes )
2. The ability to enter/view shops in the entire empire. I suggest that one city can be chosen as your main and from there you can have the option to enter shops in the rest of the empire for certain number of diamonds. Dont know how easy it would be to program but something I think people would be willing to pay for instead of travelling around for hours...