Most of the products I wish to own are already available. However, due to political climate the prices are outrageous and ammo is unavailable at realistic prices.
One area that is lacking in choices is semi-automatic belt-fed firearms. It is definitely a niche market but one that has room to grow if the law will allow it. I would like to see some quality offerings in "normally affordable" calibers, especially ones based on Soviet designs.
IMHO a WELL-MADE semi-auto version of the Soviet designed PKM in 7.62x54r would be a fun oddity to have in a collection. MarColMar sold some very limited numbers of them a few years ago. I read some poor owner reviews about the quality of the build for the semi-auto versions. However, I think if somebody made some real top quality ones (think Arsenal Las Vegas quality) they would be not have a problem selling them. Also, Inthino it would be wise to have it run with cloth belts that can be reused.
Similarly, the idea of a semi-auto BELT-FED Saiga 12 always seems to get people really excited on various forums. (I am not sure if something like this would be considered a DD, AOW or just no special classification.) A few years ago, guy put up a photo of his Saiga 12 with a belt run through the chamber and EVERYBODY wanted to have one until they looked closely at the picture and saw it was just a joke. Again, it would need to run with a reusable cloth belt and cheap plastic hull shotshells like the birdshot shells you can buy in bulk.