Use a separate table for a given "idea" record's list of applicable categories.
You can place a portal to Categories on your Ideas layout and use a drop down of Category names to control/manage what possible categories you can choose to assign an idea to.
You can see an example of this in my Known Bugs List where I assign a given Bug Report record to multiple categories. The drop down list of category names draws it's list of names from the Categories table so I can add a new category just by typing it's name into a blank portal row. Just click the "Bug Reports" button and check out the Category portal on the layout that appears.