Its a tough call a pitching coach could relaly help you out in this situation. But...I don't have all the details but Steven Ellis the guy who made this website talks about making it through a great high school team with just a fastball and a change-up. He threw good location and mastered a change. He made it to the pros.
But you also have to think you throw that hard you have talent and could easily be college bound. How many Major Leaguers do you know that throw a knuckleball along with an overpowering fastball? Not many.
You could also consider the cutter, which could operate as a begginers slider becuase if your a power pitcher the slider is a great pitch. If you could work on the chagne-up location of fastballs, then work that cutter in. Eventually you could change that cutter to a slider. Then you have an overpowering Fastball, with a slider, and a change-up. That sounds a little better doesn't it?
I belive that is exately what Santana for the Mets throws. Fastball slider change-up. Also Tim Lincecum, could be a curve but a quick breakig pitch and I think he has a change-up too.