I like the 0-6-0 sets, but why not use new locomotives?
Spectrum 2-10-2 set for example.
Using larger locomotives is cost prohibitive, and would not appeal to most people who buy train sets who usually are not serious model railroader. Also for good running they would need to include like 24" radius track which also is not practical and appealing to most people who buy trains sets. Trains sets are aimed mostly at beginners and not for advanced model railroaders, having a 2-10-2 in a set would make it too expensive for most people who buy trains sets thus it would be an unsuccessful product and therefore should not be made. If you had a 2-10-2 in the set, the cost of the locomotive alone would be a great amount, so the set would probably cost around $400-$500 at least depending on what was included. If an E-Z Command was included that would add another $100.
How about special orders? Bachmann has all the stuff already. They just wouldn't be in a fancy box.
'B" could put up an order form.
Most sets consist of loco, cars, track, and power supply.
The form could have the buyer select from a list of locos and cars. Then select from different track plans (oval, oval with turnouts, etc.). Then power supply. Accessories too.
Add it all up, then give a discounted "set" price.