It would be great to detect when a project is referencing a (non-package) DLL that is also available in a package. Would be particularly useful in cases where I might have updated one project to a new nuget version, but not another - I could end up mixing the DLL's in the output on build.

Not sure how much this might collide with actual use cases supported by NuGet where people may want to have several versions of a package installed side-by-side - however I cannot really imagine why somebody would intend to do so.