Well you can forget about Horizon making Narrow gauge. They are still sitting on a handful of Rail Power shells that to can be released but the choose not to. Bachmann would be the best call and they technically already make narrow gauge as Bachmann owns the Lilliput line.
http://www.bachmann.co.uk/price1.php?prod_selected=liliput
Well, back when Athearn bought railpower, the railpower tooling was slighty better than the bluebox tooling. Now to bring them up to RTR standards, they would have to retool them slightly.
Athearn has been doing just that. They have been slowly but surely retooling their inventory of stuff. For example, the GP50s, SD40s, SD45s, SD40-2s, Dash 9-44CW, and the SW series.
Although retooling all of the old blue box and railpower models is on their list of priorities, it's not as high priority as the Genesis series because Athearn needs to keep ahead in the race against other manufacturers.
Basically, what manufacturers will produce depends on improvements, and what the market will bear.
For example, Athearn BB was the first to make decent GP38-2s and GP40-2s. However, later on, the model was not up to the standards of the day, and so Atlas jumped the bandwagon (these engines are ubiquitous after all) and produced better GP38-2s and GP40-2s. P2K tried and sort of failed to jump on the geep bandwagon. This may be a (speculated) reason why Athearn is still selling the blue box tooling geeps as RTR.
Another example is the ACe race. Athearn wanted to produce the SD70ACe (I believe based on the Tower 55 tooling, I could be wrong) and MTH beat them to the punch, but Athearn decided to go through with it anyway because they had the "we can do it better" idea.
The GP15 was recently released by Athearn in the Genesis line. The only prior model was the Walthers Trainline (now rebranded as P1K) GP15 or the now-extinct Smokey Valley kit. Athearn made their version so that people would want to buy it because it is better than the P1K version.
Bachmann Spectrum did the GE dash 8 series. Atlas saw that they were popular but had many problems as well as cosmetic issues, so they made their own. Now, Bachmann's Dash 8s aren't popular anymore.
With the new RTR SW series engines from Athearn, Athearn saw that the blue box models needed a revamp and they retooled it based on the fact that no one else really made a decent SW1500. (with the exception of BLI, but most of the "big three" manufacturers are only concerned with each other as far as major competition since BLI doesn't own much of the market share.)
It's the great improvement and "We can do it too and do it better" mentality that gets us great products. However, it also depends on what the market will bear. If for example, Athearn made an obscure model of a whitcomb switcher or something that only one or two short lines had, then they would not make much money from them except for people who model those roads and freelancers. This is the major problem with producing narrow gauge equipment. Narrow gauge modelers do not make up very much of the market. Businesses are in business to make money and if they feel that they cannot generate a sufficient return on a certain product, they will not do it.
It is because of this fear that products won't sell that most manufacturers run on the preorder system. They need a minimum number of preorders to actually produce the model, then they produce enough to cover the preorders and then slightly more. The retailers/dealers like it as this means they won't have product sitting on their shelf gathering dust, and the manufacturers use it to gauge the market and ensure profitability, but is not so good for us as the prices would go up since the production run sizes are possibly smaller, and it also makes it harder for us to find certain models as the items may either sell out before the items are produced, or there isn't enough stock for people to buy after the preorder period closes.
