How have the roles and powers of the president been altered from how the founding fathers laid them out?
Although the Constitution has not really changed with regard to the president's roles and powers, he (or someday she) does have much more power than previously and plays many more roles.
The Founding Fathers meant for the Congress to have the bulk of the power in the government. The president was not supposed to have the sort of central role in the government that he now has. The president has taken more power over the years as the size of the executive branch has increased.
The president also plays many more roles today. In essence, the president is expected to be the leader of the country. He is supposed to keep the economy running smoothly, which is not something early presidents were expected to do. He is supposed to be the leader of his political party. There were no political parties when the Constitution was being written.
In short, the president's powers have grown tremendously since the days of the Founding Fathers.