I see what you're thinking about. Tbh I was asking myself that just a few days ago. A player like KDB, who has vision, great passing skills and is very creative could do nicely as a CM, but he would need to put on some weight for that position. Other than that, I think defensive qualities, such as tracking back and marking can be learned. But as a CM he wouldn't be able to use his excellent crossing ability which is, looking at his overall game, a very important part of it, so it would be a though price to pay.