new option needed

We need to the Category to be split to the category
1\main category: contains all my main categories.
2\Sub category: contains all sub categories for each main category.

But if the user selects one of the main category "say hardware" then the sub category will list onlyThe options for that category.

it may works like the "Assign To " option>>>that if you select "Assign To a Technician" the owner list will list the technician only...but if you select "Assign To a Queue" the owner list will list the queue list only.

So.... is there any way in future that your team will program this option for us.

Thanks.