It would be so nice to have a possibility to apply arbitrary transformations to text content of aspx/ascx files before compiling them. This will open a way for a lot of interesting code-generation techniques.
For example, it would allow for folks that don't like runat="server" not to write this attribute. Instead of this they would just be able to use a simple transformation that will add this attribute to any tag with a namespace prefix.
This is possible with a VirtualPathProvider (did it in 2007), but you completely lose Visual Studio support. That's the real issue here - we need a way to clean up the nightmare that is the .asc/px format while maintaining design-time support across the tools.