Have you ever wanted to extend HTML editor Source view or perhaps modify some of the existing functionality? Perhaps you always wanted to write an add-in or a simple feature youself but lack of extensibility support (or lack of information on it) prevented you from doing it? There is a large amount of information available at Visual Studio Extensibility portal, but I do know that we in the Web Tools team don't currently provide rich extensibility support. For example, we do not expose element tree that our parser produces and it is not easy to modify context menus.
What kind of extensibility point you would like to see? Perhaps we could publish an add-on library you could use to access cetrain internal structures? If you do not feel like publishing your add-in ideas just yet, feel free to e-mail me with requests for extensibility at mikhaila-at-microsoft-dot-com. Feel free also to ask about Design view extensibility as well.