I couldn't see building a new copy of a locomotive class that still had examples extant. In the case of the N&W J & A classes management gave up on operation of restored existing locomotives. So you would have to be sure you could find a road that would allow your brand new locomotive on the main line. The UP is to be commended for its steam program, but it is unique.

I would also suggest it would be wise to recreate a locomotive that was successful. If you build a T-1 you verify what was wrong with it in 1946. My impression is that neither the T-1's nor the J class 2-10-4's were very popular with the Pennsy railroaders who actually worked with them. The K-4 was the most liked, if I remember right. There is at least one K-4 still around.

I would push for the NYC 6000 class niagara because it was considered to be a particularly successful design in terms of performance and had a distinctly different appearance from other 4-8-4's. If the decision were up to me I would not hesitate to add modern features on the grounds that the intent of the original designer was to build a locomotive that could meet the diesel challenge. I believe his name was Paul Kiefer.