Well, off the top of my head I think the items missing the most are many that Walthers, in their infinite "wisdom" have "retired". The ones I'm thinking of were used, or are STILL used by railroads, so why all of a sudden stop making a needed item available? I'm thinking of things like the coal flood loader, transfer table (though I've heard it's a piece of crap, but that's not the point here), I think the ice house or is it the ice house or grain elevator extensions(?), rotary dumper, car float stuff, some or most or all of the steel mill structures. I may be wrong on some of these specific items as I don't have a current Walthers catalog available, but you can see what I mean.
Also, we need some (more) residential automobile garages and houses though the recent Atlas offerings are quite nice.
"Paul - The CB&Q Guy" (Modeling 1960's In HO.)