Will there ever be a Greenshot version for Linux or Mac?

Maybe you missed it, in the mean time we released an OSX version of Greenshot, more about it here

For Linux we don’t plan anything in the near future. Greenshot is written with the Microsoft .NET framework, which does not run on Linux or iOS systems. Although Microsoft released .NET Core, which is multi-platform, unfortunately .NET Core does neither support Windows Forms nor WPF, which would make it impossible to reuse Greenshot’s existing code base. As we would rather not maintain multiple code bases, there are currently no plans to create a Greenshot version for Linux.