This is similar to other features, but not quite the same. Essentially, I think making a very generic preview window that is able to display arbitrary content would be a good idea. This then allows for several things.
1. Build systems can output build results.
2. Build systems for things like LaTeX or Markdown could display their actual product.
3. You could preview HTML files when editing (possibly done through a build system).
4. Ability to view miscellaneous file types. The key here is *view* not edit. For example, I can quickly check a PDF or an image that I may have in my project files.
The viewing functionality does not need to be complex, no editing would be needed and not that many file types would need to be supported for it to be very useful.
On Mac OS, I believe this would be very easy to implement in some for with the Quicklook frameworks. This allows for simple viewing of almost all system supported file types.