>> "That would be an idea for an alternate ad network. Simply let website owners select ads, every week, to be displayed. Or in order to scale better, create pre-agreements for some brands."
Unfortunately, you have just reinvented publisher-direct deals, a mainstay of many ad networks for the past decade.
Ad networks work by letting you select categories, but not much more. Obviously that's because they want to keep control and maximize short-term revenues.