Idea Summary
It would be great to have a background page that other pages in the application can subscribe to. It would behave similar to page 0, with the difference that the page 0 is executed on all pages and this new “background page” will only be executed on the pages that subscribe to it. 

Use Case
Currently if I want to implement a dynamic action for 3 different pages I can either copy that DA to all pages (horrible for maintaining) or create it on page 0 with a condition that it only gets executed on these 3 pages. The later has the big disadvantage that page 0 will get enormously big really fast. It would be cool to create a page like “-1” that contains this dynamic action and only gets executed on my 3 pages that subscribe to the page "-1". In that case page 0 and page -1 will get executed during loading of the page.

Another use case would be that the background page is like a template containing all major fields with the “real” apex-pages being individual flavors of this page.

Preferred Solution (Optional)
Allow the creation of negative page numbers.

Negative pages behave similar to page 0 with the exception that page 0 is always getting executed and negative pages are only getting executed on individual pages.

For normal apex pages create a new attribute where the negative page numbers can get selected for rendering with this page .