I've thought about a 3-position hinged "flap". When we put stuff back there, it often slides forward into the hole, where it's a pain to crawl in and lift it out. A hinged flap that could be locked into a vertical position would act as a backstop. But there's also times we wanna use that space, so we could flip the flap outward and lay it flat. And if we just want to cover the hole flat all the way to the seats, flip it forward.This will be an interesting engineering problem. The gap can change size as the 2nd row seats are moved forward/backward. The gap proved to be very useful for my road trip of 4 people, two 80 lb dogs, and a week’s worth of human and canine luggage.
I was able to fit large suitcases into that gap by moving the 2nd row seats all the way forward, which still left plenty of legroom for my 2nd row passengers. The normal flat area of the cargo space was taken up by the two dogs.
Whatever the solution is, I hope it’s not just filler but rather a platform/compartment that keeps the extra storage space. The cargo capacity of the Gravity is fantastic.