I have a Shield on order but not in hand so I'll ask those who do own one a question. Is the semi-double stack 9mm magazine as thick as a 45acp magazine like from a 1911? It looks like it could be from pictures I've seen, but I can't be sure. If so, a 45acp version may be possible without the need to change too much in the frame area, meaning all 3 calibers would use the exact same thin frame.

Just a thought.