I would like to see Mallard and Duchess of Hamilton on the mainline.
Hear! Hear!
Also, I would like to see the National Collection's little engines out on various heritage lines. I have a book that has the engines of the National Collection in it.The trouble is I can't find it.
Out of those, the one I would most like to see is the Stanier 3-cylinder tank, perhaps working short charters on various branch lines for the benefit of tourists. Quick acceleration would be useful in staying out of the way of service trains.
Others I would especially like to see are
Hardwicke and the Stirling Single (with a full-height tender as they ran with in service). Maybe not on the main lines, but on a few preserved lines.
Henry Oakley and No. 251 would make a great main line pairing, as contemporaries of
City of Truro.
You might notice a bit of an East Coast bias, so I'll include the Midland Spinner, the SE&CR Class D, Midland Compound 1000,
Butler Henderson and
Lode Star. The Edwardian era needs more representatives out there. Perhaps
Iron Duke could visit Didcot as well, to work the broad gauge line alongside
Firefly.