I wouldn't mind seeing a an empire tier, but I am definately against making it easy. There is a reason why there was no real empire in the Middle ages. Its because nations were too strong and independent for other nations to walk over them and take them.
The English attempted to take France, and it took them a 100 years to fail. What in the world makes you think its easy to conquer Spain, England, France, Germany and most of europe to declare an empire of X?
So, I pose this hypothetical question. What difficulty, and what gains do you need to achieve before you can change into an empire? What sounds reasonable in your best guess?