It would be great if Visual Studios’s core is also open sourced and perhaps this effort is merged with Mono Develop so there mainly 1 open source IDE development effort.
There are no plans that I’m aware of to open source significant parts of Visual Studio. That said, I can see the value of what you are asking for.
Our main focus right now is in landing .NET Core. If/when it is super successful, it may influence new choices for Visual Studio. We’ll see.
With regard to .net core you have to break it down to sub-projects and communities around the sub projects. E.g. one of my interest is seeing new custom IR being supported in .net and levering .net are a more universal VM. Like wise others will have different interest. One these sub groups are defines and segregated you will see a more focused interest.