What Ray said. Last year Lionel commented that if sales of the Legacy Pacific were good they would consider releasing a Legacy Mikado in the future. I would think the Mikado would be the closest S gauge steam engine to production. Beyond that, Lionel has given no hints.
As to what is in their portfolio, A rerelease of the Legacy Challenger could be done as they did for the Y3. Lionel also has all the new tooling used for the FlyerChief 4-8-4. I have one converted to TMCC, it is a nice engine. This could be released as Legacy with minimal tooling costs.
In casual conversations with AM, there are no plans for new engines. They plan to release the revised Pacific with the Vandy tender “sometime.” When I asked specifically about a rerun of the streamlined Hudson they said it was possible but there are currently no plans to do so.
S gauge steam engines from Scale Trains are not likely. Their priority is to get the MTH HO steam engines into production. Even with almost all the tooling received from MTH, ST is more than a year behind their original plans.
Here is a custom made Reading steam engine on a sale table last month at S Fest West. It was not sold at the show. Seller wanted $600 for it. Looking closely at the engines they are all first rate and run well. It should still be available to anyone who wants to buy it.