I think a complete London Underground would sell very well. It'd be relatively easy to develop given it's mostly tunnels and there's a pretty big audience of UK players who would buy it since almost all of the UK has ridden them at one time or another. The whole system is only 250 miles in total, so in maybe 5 small, cheap DLC you could map it all out, reusing a lot of assets and expanding it easily. Plus "route hop" makes sense and it's easy to use stations as natural stopping points, unlike some maps where you stop basically in the middle of nowhere. Heck, have REAL cheap DLC and just do each colored line as an add on. Some like the green line are pretty long (40 miles ish?) But others like the Circle line are only maybe 14-15 miles. Given you can make them in half or 1/3 the time a "normal" route they'd be fast to come out with one team working on it and really cheap. Start with say the Circle line for $10 and build on from there. There's even some different in stock for different areas to be "authentic" if you want people to buy more DLC or to sell with each pack (the 1972 stock with the Bakerloo line like we have in game, but the Circle uses the S7 cars I think it is?) So you'd get a new set of cars with each DLC too and you could layer them all on one another because they can be swapped irl in case of a breakdown or something. If you want new features...well the new cars have wheelchair accessibility and you could have wheelchair NPCs to board and unboard. You can also catch people smoking/vaping in "Conductor" mode. The new station and train announcements work perfectly with the Tube of course, and you could even add some of those "Tours" they had when I was over there where they informed you about landmarks and the history of the Tube. Not sure if that's a thing anymore, but between stops it'd be a neat addition. "This station was built in 19XX and features XXX, you can get off here and see <iinsert landmark> Some of the stations if you want to go crazy are even close enough to have shops and you can walk up and see things. For example Westminster stop you can walk up and see Parliament and Big Ben and London Bridge. Great for virtual tourism.
Given DTG said doing Bakerloo Line went massively over budget with the light baking etc. for the tube sections, a replica of the whole system seems unlikely. However another line, maybe something like the Northern or Central (preferably non ATO) would be feasible - if they can overcome their anathema to doing underground stations.
It would be awesome especially given the automated announcements and a nice touch with the duel male female voice setup
