Upon opening a file, it would be nice if Revit opened up a prompt asking you to select which workset you would like to be active within the file upon opening (not just which ones would be open or closed). Often times, when collaborating, it's very easy to proceed working on an incorrect workset between different Revit sessions. The prompt would prevent this. I think a checkbox in the existing workset dialog box is not enough. It should be mandatory upon each file open.