Two products on the market do this. Pubmatic and the Rubicon project.

The last third of ad performance can be squeezed out by simple algorithmic optimization.

That's so funny how most of my ideas already exist ...

Mine might be better because I'd make it dead simple to use, and only target adsense. And I'd target small publishers, bloggers, etc.

I'm hoping I could just make some javascript the user could put in their code, and it would mix and match the adsense placements (maybe color, etc too)

It would need a way to track the effectiveness of the ads. Tracking ad clicks would be simple enough with JS, tracking how much you make would be harder. I don't know if adsense has an API for that or not.