It is the detailed parts that interest me, particularly 7mm scale. Stuff like:
Loco detailing parts, such as smokebox doors/darts, clack valves, springs, cab fittings etc.
Underframes, such as bogie sides, springs, W-irons, buffers.
Chimneys, domes and safety valves too - though have learned to turn these on my basic lathe.
The reason I say this is the loss of parts suppliers. Laurie Griffin Miniatures is in limbo, following his retirement, while the older cast whitemetal supplies seem to be ever more scarce. These parts are all fairly complex shapes. I wouldn't fancy making one axlebox from scratch, let alone four or six - though resin casting is an option for multiple copies.
Obviously drawing up the myriad of types used by post and pre group companies could be quite a task, but can't help thinking that without such small luxuries, fewer and fewer people will think about scratchbuilding. Basic body shapes are a mixture of tubes and rectangles that just need joining together it is all the details where it gets harder and this is where the growing dearth of small castings becomes a problem.