If the ball comes loose after making a catch, it is already a fumble.
Yes, a LEGAL catch. Not just a possession + 2 feet catch which is what you're proposing for a legal catch. The receiver needs to either maintain possession to the ground or establish himself as a runner before he can fumble.
You're changing that completely with your definition and again, not allowing for a time element of a catch. A receiver can catch a pass with both feet on the ground, possess the ball for a tenth of a second, get blasted by a defender and the ball is jarred loose to the ground.
It's well-known that's an incomplete pass.
Under your definition, it's a catch and fumble.
but I am waiting for a catch like the one Jared Cook made against Dallas and as he is coming down out of bounds, he collides with someone or something on the sideline that jars the ball loose.
NFL Bowling anyone?