Dynamic Custom Fields

My team would like to use custom fields to categorize requests received from clients at two levels (category and sub-category).  Currently, we have these set-up as two custom fields, but the challenge is that not every sub-category should be an option for each category. This means that the users have to know what sub-categories should map to each category.  

For example, category A has sub-categories 1 and 2, category B has sub-categories 3, 4, 5, and category C has sub-categories 1 and 5.

What would really help my team is to have the ability to type the custom fields together so that the options in the sub-category field are limited based on what is selected in the category custom field.  So if a user select category B only 3,4, and 5 show up in the dropdown for sub-category.