I'd definately say the Saiga shotguns. They are pretty much the only game in town for a box magazine fed riot gun, and are "begging" to get hit by import restrictions at some point in the future. Think of what happened with the SPAS-12.
I would also add the imported .308 caliber battle rifles. Look at all of the posts here about people who's dream gun is a FAL/G3, and how they will get one someday when the have enough money. Again, these guns are ripe for importation restrictions, "Foreign assault rifles are being imported in pieces and reassembled in the US! We have to close this loophole!"
The M1A also kind of fits into this group, being a dream gun that people keep putting off to get because it is so expensive. If another ban came around these .308 military style rifles are going to be much less common than an AR-15 or AK clone, and people will pay a premium for them.
Acutally, all of the US's military rifles will be very collectable, in fact most already are. The one thing keeping the prices down is the CMP. When they run out American rifle prices will jump quite a bit.
Some foreign surplus rifles will also become more desireable as supplies dry up. The most obvious will be the K-31, as it is a unique design and very accurate. I would also dare say that some of the Mausers will also gain value, as they are somewhat iconic, especially in dangerous game literature because of the controlled round feed.
The surplus semi-autos are also a good bet for increasing their value, although the time to get the is probably past. An exception is the SKS. When supplies run dry or import restrictions take effect there is a good chance SKS's could double in value. Mind you we are talking a $200 gun, but there are numerous examples of "plinkers" which shot up in value when the supply dried up, for example the M1 Carbine.
I didn't exactly follow the "Currently Produced" guideline, but all of these rifles have a steady supply, "new" guns are still flowing into the market. This is in contrast to the current collectors pieces which have a fixed amount in public circulation, in which case someone has to sell one for you to buy one.
As to what is currently being commercially manufactured that will become a collector's item? I'd say there are two potential collectors pieces. The first is liked by nearly all of the shooters who use that particular type of firearm. An example of this is the Colt SAA. The other type is a gun that never appealed to the majority but there is a significant minority that love it, making a cult gun. .44 special revolvers suitable for carry, like the S&W 624 are an instance of this phenomenon.
Lastly, as has been said before, any gun that is banned is going to rise exponentially in price.
