Well, here goes:

Loco: New body class 25 (low hanging fruit, surely?)

Then it gets a bit more blue-sky...

Coach: RTC Trib train auto-trailer

Wagon: John Summers iron ore hoppers

Given that there have been two versions of the RTC 24, and the somewhat fictional 28, there surely has to be a market for the trib train coaches?

And the John Summers wagons, which are similar to the ICI limestone ones, are a memory from childhood. I have no expectation that I'll ever see them in RTR. But it is a wish-list.