I know it's probably not likely and I'm not sure if it's even possible but one train I would love to see is the 20th century limited!
The NYC Water Level Route (or a 50-100 mile section) would be fantastic. I'd love to see NYC's famed Hudsons hauling the 20th Century Limited at speed. Perhaps we could get the Dreyfuss Hudsons, the Commodore Vanderbilt, and of course, the NYC Empire State Express Hudsons, as well as the standard J1 and J3 Hudsons. For freight services, we need an NYC Mowhawk. A Niagara could be included with the NYC route, too (or added as a future DLC). A few smaller steam locos could also be useful for yard switching, local power, and commuter train operations on the route.