Improve visualizer API
The current API for writing custom visualizers is horrible. You have to apply an attribute for the visualizer to the type you want to visualize, meaning that you cannot write custom visualizers for anything in the BCL or 3rd party libraries.
As a result, there's not a single normal visualizer available as an extension in the Visual Studio gallery. If that's not a sign of failure for this extension point, I don't know what is.
Custom visualizers should declare what types they can work on themselves. This would be far, far more powerful in allowing the community to build visualizers for all the common types (a better DataTable visualizer anyone?)