Piccolo definately needs a speed increase. Whenever he would get one of his power ups, he would become one of the fastest characters ever. Of course the saiyans had a gazillion transformations and Piccolo ran outta people to fuse with, so in the end he got the short end of the deal.
So:
Speed his ass up.
He also should at least get the nail and kami transformations. Both of those fusions were extremely important to dbz. Without Nails, Gohan and Vegeta die. Without Kami, well, i guess not much changes, but he is whole again.