I've been following the thread with interest. It seems to me that price is a major concern for most, especially when you consider that you're going to be wanting in the region of 10 or more 4 wheel wagons to make a set.
Keeping the costs down all comes down to how quickly it can be made. With this in mind, especially in the case of RTR, you are almost certainly looking at using a modified UK chassis of some sort with a custom built body or top. Every process you can cut brings the price down and makes the project more and more viable. I understand that if may look easy on the outside to just widen the frames slightly to cater for 21mm but this might mean designing a new part or a scratch-built chassis instead of a donor. This means that the 99% have to pay more so that it is more convenient for the 1%.
I have to agree with Anthony on this one. We've been building models for the last couple of years and have had 1 enquiry about 21mm and zero orders.
The economics simply aren't possible to make a 100% perfect, 21mm ready, RTR wagon for less than 30-40 euro a wagon and about 20-30 for a kit. The absolute cheapest I could see a RTR wagon being is about 30 euro and 20 for a kit and that's using a UK chassis.
I'd also caution that more competition is not always a good thing in a tiny market as it can force quality down in pursuit of price reduction.