It's doable,and I'm sure there are plans for one in the future. My problem with it would be the reduction in capacity with the larger round while still being in keeping with a compact slim auto.It would have to be wider.
I don't put as much premium on capacity in a pistol as most people,but I sure wouldn't want to go less than what the .40 Shield carries for a somewhat larger round,that may not even perform as well from the already short barrel.
If a .45 version was much larger than what the current Shields are then it would be something else all together.
Then again,today's engineering is marvelous,but you can only cram so many good gifts into a small box.
I'm a 9mm user,and for me the current Shields are perfect for their intended market use.