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

Probably not, at least not in the near future. Greenshot is written with the Microsoft .NET framework, which does not run on Linux or iOS systems. Of course there is mono which allows runnning .NET apps on operation systems other than Windows, however it 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.