Hi, Rajen Kishna here, Product Manager on Visual Studio for Mac. Our goal with Visual Studio for Mac is to create a native IDE for Mac users with workloads that make sense on macOS. That means "desktop app" development will target macOS and Visual Studio (on Windows) can be used to target Windows.
The core of the IDE definitely has a heritage in Xamarin Studio, but this release has brought in so much more with .NET Core/ASP.NET Core development for web apps/services, Unity support for game development and cloud integration with directly publishing your web apps/services and previews of Docker and Azure Functions coming very soon.
Extensions is definitely another area we're looking to align more over time. Currently, there is an extensions framework, but you're right that it's different from the one used on Windows.
Definitely keep the feedback coming, we're listening and looking to act and prioritize accordingly!