As for locomotive and since there is work on a T-12 already under way, I'll choose a D&RG 4-4-0 (a la') Eureka era.
For a non-D&RGW locomotive, I'd like to see ex-GTL 40 and 44 doublehead up Cumbres. Closest to the C21s and/or C25.
If you are discussing rolling stock, I'd like to see a replica train set of the San Juan. How about a replica of the 6526 or 6527 for freight equipment?
How about a replica of the early 2-4-0 (operating or static) with a couple of 4-wheel cars that were used very early in the 1870's on the Denver-Colorado Springs-Pueblo line.
And if you really want to reach, how about Telluride's goose in "regular" service on the C&TS or D&S instead of rusting away.
Any takers for building Galloping Goose #8?
Doug vV