@vesuvius:There is no Visual Studio Online anymore. Branding change. It's Visual Studio Team Services now. :P
Visual Studio Online (err, Visual Studio Team Services) has never been an IDE. It's still not. That's actually why the name change was made, because the name sure sounds like it means a cloud hosted IDE. I have not done anything on Linux in far too many years now, but I believe MonoDevelop would still be your goto choice there. AFAIK it's been updated to handle all of the changes in the .NET ecosystem, i.e. dnx, NuGet, .NET Core, MSBuild, etc.
NuGet was a game changer, but I think the real game changer is open sourcing .NET and creating a portable core (with NuGet for all libraries instead of needing to install a monolithic framework). The pace of (very positive) changes in .NET since they started that has been astounding.