Not sure I completely agree Allan.
There were a (very small) number of S still running in the early 60s (heck, even a Q at Murray Bridge IIRC!).
The other loco that might be of Interest would be the 2nd F given the limited brass kit run ( and that you really had to buy another chassis kit).
With MDS "soon" to release their 500 resin body and the 520 mechanism thereafter (once Nigel stops stuffing around with that 720 chassis the next most popular I'd imagine would be a 620 (although RTR 7xx or 520s would likely find a market). The 600s are probably a little too specialized although I'd buy one. So you start getting to things like the 2nd F and S.
On the DE side there are plenty of 830 options and decent 930s to be had. The 700DE has been done and is getting a little too new as you said. Perhaps a decent 900? The 500DE market is a little saturated although not with top notch offerings. 350s? Can't see huge interest in 800s.
Anything else I'm not thinking of?
So, subliminal message time : 620!
I have for years been fairly impressed with the Webb era heavy power, & seeing photo's of the 720 it strikes a bit of a chord with me, as do some other loco's as well.
I know of some U.S modellers who also feel a bit the same way with them & a couple have said the 720 & some of the others would not go astray on their layouts, I guess they could find a niche type market in that scenario, also if they were produced in RTR form, likely also depending on the price, they would sell ok also. The thing is that many modellers also do not necessarily just stick to a specific era as much as before, so perhaps some SAR modellers may be inclined to extend the years a bit in order to incorporate these types of models.