One problem with your idea is, what constitutes a "work" for the purpose of renewing copyright? Currently, a single photograph and a two-hour movie that cost $$$ to make are both "works". Charging $5000 to renew the copyright on an individual photo will bankrupt people who make a living doing stock photography, but it's peanuts for a large movie studio. You could create a category of "small works", like individual photographs or short stories, that can be batched together so that you pay only one fee to renew a group of them, but flat-fee-per-work under the current definition will cause problems for some classes of individual creators.

Personally, I think we need to tear the whole thing down and start over. Base copyright on individual works on a "use it or lose it" system—as long as copies of a work remain available for purchase (not rental—streaming or DRM-gated access is not sufficient) from the copyright holder at a reasonable price, they have exclusive rights to it. Stop publishing the work, and it lands in the public domain within 5-10 years. This needs to be accompanied by substantial reforms on how derivative works and trademarked characters are handled—we need a universal mechanical licensing system with a central clearinghouse that allows anyone to create a derivative work for a flat fee or percentage of revenue, and an official, legally-binding system for indicating "this derivative work was not created by the trademark holder". (Figuring out how rights on unpublished works function in this schema is something I still need to work through, but they're not a major concern for 99% of people.)