In my workflow every step is logically "owned" by a Project Role ( eg: the step "Test" is logically owned by Project Role "Tester" and the step "Develope" si logically owned by the "Developer").
When I change status to my issue I want only to see the owners of the next step in combo Assignee (eg. When changing from Develop to Test I want to see only the users having "Tester" as Project Role in the Assignee combobox).