Make the ideas category field a multi-step selection of the top level category then the sub-level category

Due to the nature of our work, our parent category and sub-category lists are long. It would make submission of an idea more of a streamlined process if we were able to have a field for selection of parent category and then a subsequent field for selection of sub-category based upon parent category selection. While this is possible with a pre-defined choice list custom field, the sub-category will then not be displayed within the ideas portal on the left side.