The metropoliton line has the most pedigree out of the LU lines as it is the oldest and reaches the furthest out of London. I think it would be the best of the LU lines to add, it's not too long, with a full stopping service taking about 90 minutes, and there's a mix of fast, semi-fast and stopping services, as well as a mix of overground and underground ares, and there's areas where it runs along side the Jubilee Line, more potential for 1996-stock and a Jubilee line extention. For the unit, I think the old A-stock would be really nice to have, but I imagine the S8 (or S7+1) would be easier to implement as they're in service today and modeling the S8 stock could lead to future Circle, H&C and District line routs, as they all use variants or the S-stock, maybe the old A stock could be potential DLC for a classic mode maybe? which could be determined by which stock you choose when selecting the time table. I think dovetail introducing The Bakerloo line is revolutionary for their train simulation, I think they should really exploit this to really create some quality metro/tube routes
If they do add another tube line I hope it is the Met, or at least a cut and cover one. Another deep level line would be a bit too similar to Bakerloo
I think Met or district would be best,, I personally would prefer Met because I used to get on it a lot
