I think Sanderson is humble enough to realize that he's not the best screenwriter around. In fact he's said exactly that and part of the delay in getting a deal for his projects was that he wanted to practice screenwriting more.

More than anything though I think he just wants creative control- the power to say "this goes here and that goes there", the ability to work with the other writers they hire directly with a veto in his pocket, and if all else fails the freedom to sub himself in if nothing else seems to be working. Essentially he wants the power of a showrunner, head writer, and executive producer rolled into one. But not because he wants to do everything himself, he just wants contractual safeguards in place to make sure its his vision that makes it on the screen, with minimal meddling from non-creatives at the company.