Can you explain how your meta-browser should differ from an ordinary operating system? I.e. additional functionality it would have when not compared to a browser, but to the substrate browsers currently run on.
In a nutshell. It would only be a simple shell, with extremely minimal API (for user settings, etc), which could sandbox browser engines. This (a) makes "switching between browsers" a seamless experience to the end user, (b) makes the development experience much more friendly to the developer [since they're developing for their browser engine of choice].
Dear god why. What is the point of using the web if I'm going to target specific browser engines only? I may as well write native apps and target specific OSs only.
If your idea were implemented, I'd expect that in 30 years someone will propose the meta-meta-browser so developers can choose the meta-browser that hosts their chosen browser engine.