I have an irregular "metro-style" grid of items of various types: Blog posts, press releases, reports etc. To the left, the user should be able to filter the grid by Topic and Content type.
The topic list is however growing (it will reach a max of 20 items), each belonging to one of three overarching themes). As the mockup shows, I would somehow like to show the theme headings even though only topics are selectable.
What would be a better way of allowing the user to filter by Topic and Content type, still showing the theme headings somehow?
Replacing the current with 2 dropdowns? Dropdown for Topics and radio buttons for Content types? Other?