To do this you don't need a lot. I've hinted at this kind of stuff before with my Lookup Select Attribute post. Here I'll spell it out.
Make
categoryvariable with the following bits- Type: Lookup select box
- Include none: true
- Table:
sys_choice - Reference Qualifier:
name=incident^element=category^language=en^inactive=false^ORDERBYsequence
Make
subcategoryvariable with the following bits- Type: Lookup select box
- Include none: true
- Table:
sys_choice - Reference Qualifer:
javascript: 'name=incident^element=subcategory^language=en^inactive=false^dependent_value=' + current.variables.category
(note the dependent value column in your query) - Attributes:
ref_qual_elements=category