Everyone in Naruto says that the Rasengan is at the "peak of shape transformation." Explain to me why couldn't a person make a shuriken out of Rasenshuriken. Explain why you couldn't create a Rasengan: Sharp Spear. Explain to me why any user of the Rasengan with really high chakra levels couldn't create a Susano'o-like structure.
Basically, the question is why can't you shape the Rasengan? It's supposed to be the best of the best in shape transformation, but it's a sphere. It's probably easier to create a spinning ball of chakra then a dragon made of chakra. So, why can't you make different shapes using the Rasengan?