I doubt they'll make any one-off or uncommon steam locos as their use in timetable mode would be really limited. I'm hoping for a Black Five on a busy 1950's era route like Somerset and Dorset or (part of) North Wales Coastal. Going big with the first steam DLC seems like a better way to show it off than having just a single loco DLC on a heritage line.
I'm probably wrong, but having just watched a Youtube video on the subject, I'm sorta hoping it will be the Metropolitan or District Line that sees the first steam train.
Still very specialised though. Really it could be just about any UK section, past or present. However the one thing it will not be is Thomas. This is not a kiddies sim and down that road lies a licensing nightmare. Even if that could be overcome, you would have to build the world from scratch. I’m not certain what processes the UE4 editor employs, but it almost certainly uses DEM data for terrain and some sort of mapping overlay the guys (and gals) can follow, neither of which are available for Sodor.
LB&SCR E2 steam locomotive Thomas BML then you have a GWR 5700 Duck GWR 1400 Oliver complete with a toad brakevan on the back or an Autocoach Alice/Dulcie out of London Paddington to reading GWML steam era. Remember if they start to do a challenge only Jaime has an advantage over because if you are subscribed to the official YouTube channel you're going to see him time to time pull out a steam locomotive. Want an Italian steam locomotive you are looking at FNM 200 Gina steam locomotive. That one can run to the start of a Rivet Games route the Gotthardbahn Chiasso station on SCMT RS4. After Chiasso it's 15kv 16.7hz 1.45m pantographs ETCS Integra Signum ZUB123 Sifa all the way to Zürich via Zug Lucerne and eventually Basel including the massive Rangierbahnhof Basel Muttenz via Aargau. German steam locomotive DB BR 010 Bamberg Hof Railway or pre 1954-1957 Main Spessart Bahn