I recently saw the CodeLens "Code Information Indicators" feature in Visual Studio 2013 Ultimate and thought if it would be easy to implement this feature as a PlugIn for the code editor.
This feature shows how many times something refers to a class/field/method. It also shows version control information and test results.
I think the PlugIn API already provides the usage/reference information as the IDE tracks the usages/references (Menue: Edit > Find > Show usages...).
What I don't know is, if it is easy to show these gray "ghost" lines in the code editor ("0 references..." in the screenshots) and maybe add a hyperlink to them.
What do you think?