Description of problem:
Assuming I'm understanding the flow of the installer correctly...

Currently pages need to know what comes before or after in order to navigate correctly.  This leads to some limitations like plugins adding custom pages in custom locations in the installation flow.  It also means that adding a page in the core installer can cause some headaches since you have to edit both the page forward and the page back to make sure the flow is right.

A slightly different method for doing this would be a top level process that makes pages available to the user.  That top level page knows the order (in some method where a plugin could add a new page in between existing pages).  When the user completes a page, it returns a code like "forward" or "back" so the top level process knows to proceed to the next or return to the previous.