Welcome to MRF but I think there are a number of problems with your idea. The first of these is in establishing that demand: some limited editions (e.g. the Bachmann Collectors' Club Ivatt 4MT 2-6-0 in unlined black) have sat on the shelves for months and, where they have sold out, command second-hand prices little (if at all) above their selling price, whilst others (e.g. the Modelzone Southern 171) are highly sought after and fetch prices well above their initial selling price. Whilst there are often similar variations in demand for models in the standard ranges, I should imagine a manufacturer wouldn't want to risk adding another model for which demand is uncertain: the attractive aspect of a limited edition model for the manufacturer is that the run of models is already wholly allocated and will be swiftly paid for, leaving the risk of unsold stock to the retailer and removing it from the manufacturer's domain.
Moreover, I should think it would be a hardy manufacturer that would want to sour trading relationships with retailers: if a retailer has been plugging its exclusive commissions in the press and to its customers for some time (the best way, in all probability, to gauge demand), being told that the manufacturer wishes to produce a further run of the model which you have commissioned (and possibly researched), thereby making your exclusive worthless (who'd want to buy your limited edition when there's a lower-priced model of exactly the same prototype in exactly the same state), would no doubt be frustrating. Even producing a run after the retailer has sold out of models would be dangerous. Bachmann produced a Shanks two-tone green Freightliner 66 in their standard range some time after Kernow Model Rail Centre commissioned one, which sold out and subsequently commanded very high prices on eBay &c. and although the standard one differs slightly by having additional logos and digital sound, I remember that there was some anger from people who had purchased the original model either from Kernow or second-hand and whose investment was, in a way, wasted. Offering the same model soon after would not only result in customers becoming angry at the manufacturers/retailers giving them the short shrift with regard to limited editions but might actually reduce the popularity of limited editions (if you thought that a limited edition was going to be followed up by a standard model with the same features and the same specification, would you buy it), resulting in fewer commissions and fewer projects (which may or may not eventually be produced as standard models) coming to fruition, leaving the modeller or collector with no models instead of 500 of them. There will often be cases where demand exceeds supply but I think that the alternatives (with the possible exception of the NRM's idea, whereby the very limited edition has extra features, although that increases the cost and may make the idea unpalatable to the retailers, upon whose shoulders, methinks, any further runs of their commissions will rest) are not any better.