"I am okay with "what if's" for those wanting to add to their favorite road rosters. (and even fantasy colors too) [Is the green Southern RR Light Mountain a fantasy locomotive?]"
No, not a fanyasy - the Southern actually had 2 classes of light 4-8-2 - one earlier that was not USRA-based, and the USRA version (which had some modified externals - like running boards - later). Both classes (Ts and Ts-1) were painted green, and were often used in the same sort of service as the more-famous Ps-4 Pacifics, but were a bit more powerful. (They may have had a lighter axle-loading number, also? More axles.) The SOU did not run these here (Mobile), but the L&N did run their USRA 4-8-2's into town - often at the head of the Crescent Limited, which was a joint PRR-SOU-A&WP-L&N train from New York to N.O.
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My understanding is that the Frisco 1500's were based on the USRA design - maybe the Heavy - but configured differently, specifically having a lower-mounted boiler, which gives it a different, taller-domed look. But, as said above, Lionel's Mountain could make a very reasonable stand-in, especially with a few carefully-chosen changes, unless you want to build one. We have one of these in Mobile, with the wheels painted an unfortunate red (he saw too many Chinese 2-10-2's - and he won't listen; a dangerous type of "railfan".)
The USRA Heavy and the USRA Light 4-8-2's had the same running gear and "engine" - except the Heavy had slightly larger cylinders. Here's hoping that Lionel shows up with a fat-boilered hybrid Heavy Mountain on this frame. Those were big engines.
Most any large modern 4-8-2 model could employ this frame and mechanicals, even if it meant different drivers and trailing truck (that's easy). Valve gear? That need to be changed when necessary, though I do not think that Lionel did this with the latest offerings. (For example, the L&N needs Walschaerts, but the model has Baker, I think.)