Rectangular wagons yes....but please, no more salt wagons ! I think the RTR guys have produced more than enough salt wagons over the years. Surely they were not that common ?
I think we could use Hydras, Serpents and Loriots (and similar for other railway companies). I think I read somewhere that this type of wagon was once as common as the 5 plank open wagons but they are not seen as much in the modelling world. RTR seemed to have ignored them which is surprising given that there is enormous scope there for producing the various loads these wagons carried.
I also agree about sticking with pre grouping era wagons as these hung around for a long time.