Google has published Chrome Extensions Manifest v3 for public view. Google uses manifest version as a method to restrict certain capabilities of certain extensions. It's a controlled method to test features as well as phase in and out features. This manifest is available for public view, but still subject to change. Coming changes to the Chrome Extension platform focus on increased security, performance, privacy and ergonomics.
The first major change in Manifest v3 is to the Background process. The only permissible background presence will be Service Workers. Service Workers will eventually replace many of the different background processes that have developed over the years (e.g. background pages, event pages, etc.)
The second change is to is to web-accessible resources. All web accessible resources need to support dynamic resource URLs. Further, resources committed in a non-extension context will need to be specified in the web accessible resources. Dynamic content scripts will provide more capabilities to extensions.
Finally, Manifest v3 lays out some API changes. Blocking capabilities in the WebRequest API will be restricted (declarativeNetRequest provides an alternative to blocking). The browser action and page action will be merged into a single, action API. Chrome favicon capabilities will migrate to a new chrome.favicon API. The various capturing capabilities available through tabs, pageCapture, tabCapture, and desktopCapture APIs will move to a single capture API. Any API that's functionality is available on the Open Web Platform will be removed. All APIs will be updated for support of ServiceWorker-based processes. Deprecated APIs will be removed, and other APIs will be enhanced for performance, utility and ergonomics. For more information, visit the Manifest v3.