I'm probably going to get blasted by some folks for this one, but...I'd really like to be able to open a card window within a card window.
When FileMaker 16 introduced cards, I completely redesigned my solution's UI. Without going into a lot of detail, my tool is a glorified task manager. The user creates a "browser" window and can set various filter criteria to see upcoming tasks in either a list or calendar view. The user can open as many browser windows as he or she chooses.
Tasks open as cards inside the browser window. Pre-16 we allowed users to open multiple task windows and, while very flexible, this created a lot of problems, not the least of which was window clutter. The browser and card approach has streamlined things very nicely and users can still open more than one task side-by-side if necessary. They just need to open a second browser window to do so.
I'd like to be able to open another card within the open task card window. I have a custom date picker than opens as a pop-over because I cannot open it as a card. I'd like to be able to use cards for other things, like editing an address or entering shipping information for a shipment we're expected at our warehouse. Since the main task window is already open in a card, I cannot use cards for any of these things, hence the request for card within card functionality.