I agree that it would be doable, and that the deerfield platform would be a viable one. My concern, which is probably what has kept Ruger from doing it already, is this:
The Deerfield already costs over $500 as far as I can see. This is steep for a plinker (unless it's the versatile and omnipresent AR-15) that will always cost more to shoot than carbines in 223 and 7.62x39mm. The thing that moves the deerfield is that it fires a well loved brushbuster and well reputed deerstopper. Those using 357 for hunting would use it as a backup to a rifle or are in the vast minority who hunt with them as a primary arm. $500 is usually something people will only justify for a rifle used for a major practical purpose.
Ruger would be spending additional money developing and tooling for a rifle whose sales will never approach those of the 44 Deerfield. I'm not even sure how well that carbine is doing, as Ruger discontinued the high quality PC carbines in 9mm and 40 S&W.
If I were to suggest a basis for a 357 semi-auto carbine, it would be with a recoil operated action, technologically similar to the 10-22 and Marlin camp carbine but with a superior recoil spring mechanism. If they did use a rotary mag or a short double stack one, they would have an appearance more acceptable in the sporting market than the protruding pistol mag of the retired Marlin and Ruger carbines. I'm not saying this out of fear of capacity haters, just that a protruding mag gets caught and is just in the way on a rifle whose shooter is trying his best to make a one-shot kill and would never find any more than two or three follow up shots useful at all.
Those who really want to shoot 357 indoors, as opposed to the regular pistol caliber carbines, still have leverguns to go for. That's another issue - If a semi-auto 357 was the only 357 rifle on the market, it would probably stand a better chance, but most of the folks who like 357 for hunting would also like a levergun more than a semi-auto.
Now I'm not saying this at all to say I don't think a 357 Carbine is a great idea. I just think that it would have to be obtainable for under $400, have a simple, rugged, and reliable action, keep an attractive sporting profile, have cheap options for optics, and have a strong market already lined up for it. It would cost tens of thousands or more to develop most likely, so say the profit margin is $100 or so, they would have to sell a thousand before they have any return on investment. That would take a very long time if they didn't have a long sign-up list. This is the kind of thing that would need pre-orders in order to produce. Big companies couldn't touch it, because it would be too big a step from their regular product lines and too small a quantity. More specialized ones like Thompson (which is now within S&W) would do it well but would charge $600ish, have limited options, and lots of high grade features that people would either love or hate, and most wouldn't want to pay for either way on a 357 semi-auto.
It would have to be a smaller company with the capability to do quality sporting arms. Most small companies focus on defensive arms. This would take something new, an unprecedented combination of talents, economics, and earnest marketing.
Our firearm industry needs something new.
