If I were doing a 1/32 kit I would want to hit the middle of the road in terms of appeal eg 0-6-0 tank
So the ideal would be a kit requiring few castings, all of which could be used for multiple locomotives classes (thinking of future possibilities), and preferably with multiple-region appeal. Well that's easy enough - with LNER tank classes J65, J66, J67, J68 and J69 you're aiming at six birds (trust me, in this case five equals six!) and dozens of variants thereof, with one fell swoop. All of the classes have a family likeness but are also visually distinctive from each other.
From the mid-20s they were diverse in their allocations and spheres of work, and could be found not only on the GE lines in East Anglia and London (obviously!), but, for example, on the ex-LBSC lines (via the ELR), Immingham, Lincoln, Frodingham, Barnsley, New England, the Mersey Railway, Boston, St. Margarets, Staveley, Kings X, Hatfield, Hitchin, Louth, Grantham, Sheffield, Doncaster, Leicester, Gorton, Trafford Park, Liverpool, Wrexham, Eastfield, Parkhead, Perth, Dunfirmline, Carlisle, Dumfries, Burntisland and Kittybrewster and oelsewhere. Some were paired with tenders in Scotland for light-branch working, others went into industrial use, and a few were bought by the WD.
For the sake of argument, if you chose the basic shunting versions of the 66-69 classes the castings/turnings list would run to: Buffers, smokebox door + handles, chimney, dome, ross pop valves, blow down cock, detailed backhead, tank filler lids, clack valves, reverser lever, sandbox lids, brake standard, whistle, gauges, sight feed lubricator, blower valve, injectors, handrail knobs, couplings.
Another half dozen or so castings (bufferbeam pipes, westinghouse pump, vac ejector, vent pipes, westo cylinder and reverser wheel and you're ready for the passenger variants of the J tanks.
As the GE, was at the forefront of standardisation, all of these castings would be suitable for other GE classes (in therms of castings you're more than halfway to a J15 0-6-0 and an E4 2-4-0, and more than three quarters of the way to the F4/5/6 2-4-2 tanks).