Remember the CS40/45? The .40 version was produced on the .45 frame/slide, not on that of the CS9. Hard to see them stuffing the ACP into the 9mm-sized Shield 40. A Shield 45 would require a new frame/slide design, I'd think.
If S&W invests the effort for making a larger version of the Shield chambered in .45 ACP, it would have to sell well enough to justify taking the machine time away from the calibers that ARE selling faster than they can be produced.
The ACP would be enough of a gamble, but a GAP? It's shrinking for LE sales, and even Glock isn't advertising a wealth of either LE or commercial sales.
Okay to day dream, though.
Last time I asked someone about a Shield 45 being released anytime soon, I was told not to hold my breath. The guy said he'd not heard of hints of a Shield 45 project, although that didn't necessarily mean anything.
Besides, a couple years ago I was told about ongoing development of a SW1911 chambered in .40 S&W, and it was thought it might see release soon. Haven't seen it, yet, though. I could see a SW1911 in .40 selling better than a Shield 45, too. I rather suspect the market appeal for a 5+1 shot subcompact .45 might be a bit less robust that some .45 aficionados might wish.
As much as I like my little CS45, if I could only keep one, I'd keep the CS9. A 7+1 shot subcompact 9mm is handier, and easier for most people to shoot, than a slightly larger subcompact .45 that only holds 5+1 rounds ... and I'm a long time .45 enthusiast & owner.
We'll see, though. I suspect it'll come down to market surveys and consumer demand (including any LE demand for a diminutive subcompact .45).