You do realize that a variable ND filter if even possible on the Mavic would 1) be too heavy for the Mavic's lightweight gimbal, 2) be too thick that it may create vignetting for the Mavic's somewhat wide 28mm lens, and 3) would be too costly and thus not be in demand for the average Mavic user that it wouldn't be a worthwhile doing the R&D on in order to produce them to begin with. Yes, you can probably cut some costs in making some kind of variable ND filter for the Mavic but that would also sacrifice image quality. They would have to employ some seriously exotic glass and materials to make something that is high quality, lightweight, small, and functional... and it would end up costing more than an entire Mavic drone so nobody would buy it.