McKinley had planned a more progressive administration in his second term than in his first. We probably would have seen something very similar to what we have today, with the presence of Vice President Roosevelt clearly felt. We may have seen the Progressive Revolution move a bit more slowly, but overall, the differences would be pretty mild.
I've always been quite a fan of McKinley, by the way. Good man, excellent President. He knew how to unite people and he allowed the US to rise above the petty politics of the Gilded Age. Had he survived, he would be remembered today as an American Hero.