A few week ago, I travelled to GDC and MIX. While in planes, airports and in spare moments in the conference, I implemented a feature I've wanted for some time - integrated T4 templating. This takes the T4 engine that I wrote for ASP.NET MVC templates, and exposed it within the IDE as a "custom tool", like Visual Studio does.
This also meant I had to implement a simple version of VS-style custom tools, or generators. Set the file's "custom tool" property to "TextTemplatingFileGenerator" using the property pad, then whenever you save it, MD will run the generator on the file.