I have a product on Amazon currently. What sellers really want IMO is conversion tracking on Amazon's platform. Even the ability to add your own Google Analytics tracking code would be amazing. This way you can know how many visitors your pages are getting, where they are coming from, and if you buy paid traffic, how that is converting and whether it is worth it. Without this, you're shooting in the dark with marketing spend.

App Annie is great, and I've used it for general analysis, but I think what most sellers want is deeper analytics so that they can track marketing spend with conversions. This is something no third party can do. It'd need to come from Amazon.