Let's get back to basics. What does FF actually mean? It means, the same format as 35mm film, nothing more. Why 35mm film? Because Leica decided using readily available 35mm movie film stock would be a cheap alternative to roll film despite a loss of resolution. There is nothing "full" about it. Given that all lenses and I mean all, are circular, the only format that could truly be called "full" is circular. The nearest available aspect ratio to circular is square, then 4:3, then 3:2 and so on. So in terms of pixel count versus gear size, square is the most optimised and therefore the only available format that deserves the title "full". Nevertheless, the term FF is now firmly entrenched to refer to the 35mm format. So, ideally, all sensors should be square. One could then crop to whatever aspect ration suits the composition either in camera or in PP. It's just a matter of taste.