This needs more detail to make it clear what you are wishing for that will not happen.
It seems like you're thinking of a specific application, or at least use-case. Can you elaborate?
Once you're launching an application, it seems like the application can negotiate with the external site directly if it wants.
#1 use case would be a password manager. It would be best if the browser plugin part can ping say, the 1password native app, which runs locally on your pc, and say "Yo I need a password for google.com" - then the native app springs into action, prompts for biometrics, locates the password or offers the user to choose, then returns it directly to the browser for filling.
Sure you can make a fully cloud-reliant PW manager, which has to have your key stored in the browser and fetch your vault from the server, but a lot of us like having that information never have to leave our computers.
Extensions can already use a better mechanism for this (https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...) than starting a local web server.
Browser extensions play by very different rules than websites already. The proposal is for the latter and I doubt it is going to affect the former, other than MAYBE an extra permanent permission.
you missed the point. password managers are one of the many use cases for this feature; that they just so happen to be mostly implemented as extensions does not mean that the feature is only useful for extensions