Almost nothing fits under the seat in front of you in real life if you look at real world seats. Struts, boxes and your own feet already block this space. Maybe a small, half empty, soft backpack might fit if you are willing to carry it on your back after having stored it in a very dirty place. This is why size limits for carry on are needed to put everything in the bins.
Bins are a problem since more seats were squeezed in the existing cabins by moving the aft pressure bulkhead back, rearranging galleys and lavs and reducing seat pitch. I see this as a big failure in modern cabin design. Cost per seat went down but how many departures get delayed day by day over this stupidity? Bins must be made much bigger. Forget fancy mood lights.
An easy solution would be to sell something like a small, reusable, cheap Ikea bag sized the exact dimensions permitted. Passengers could buy one and put their carry on in - if it is legal sized. You would just need some individual lanyard or decoration to find your own bag again.