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This is a feature that ALL but 1 of the many help desk systems do not have.
While simple looking, Turbine, Inc. has a great example of how to do this well.
How it works:
1. A user selects a support topic from the dropdown (eg. PC, Mac, Android)
2. A list of sub-topics automatically appears in a new dropdown to the side of or below the previous dropdown (eg. Hardware, Software)
3. From there another set of nested sub-topics appears with a new dropdown (eg. Computer won't turn on, Making a wierd noise, Can't insert disc).
Eventually we end up with
-> Computer won't turn on
Dynamically allowing for unlimited descendant topics until the user reaches the end of the chain, and possibly suggesting knowledge base items that match the end choice.
This allows admins, managers, and auto-assign to more accurately assign cases to the relevant people, and prioritize importance level.
My request is that you use the existing v1.7 topic implementation, but allow for unlimited parenting of topics & subtopics, with dynamic drop-down menu scaling based on parenting.
See Turbine, Inc support for the example. They even have more information options that dynamically pop up in order to gain more information from the user. http://support.turbine.com/ics/support/ticketnewwizard.asp?style=classic