Add ability to create a content blueprint (a block of content which is editable within another page)

As a confluence user I would like to write a block of content that I could use within other pages, like the excerpt include macro, but also edit that block of content within the current page.

An example of how I could use this feature:
1. I write best practices for Product X (in template format)
2. I write another page for Client Requirements for Client A, client A wants to use Product X so I include the best practices for Product X in the Client Requirements for Client A page and edit the details of the "Best Practices for Product X" to have specific details for Client A
3. I write another page for Client B, client B also wants to use Product X, so I include the content and modify for specific details for Client B, etc.