.Net is going cross platform. What are you going to use if you develop on OSX or Linux? It won't be VS
Visual Studio is heavily tied to WPF (Windows-only) presentation framework, COM (Windows-only) for communication across components and plugins, as well as bits of native legacy code throughout (Windows-only.)
Truly, you're looking at rewriting Visual Studio to get it on OSX. Heck, MS would probably be better off just buying Scrawl and rebranding it as Visual Studio for Mac.
And remember: while .NET is going cross-platform, there are no public plans for a cross-platform UI toolkit; there is no cross platform WinForms or WPF from Microsoft.
Essentially, this is what they would make. Or, they could simply buy this...