The Bakerloo line used to have 6 extra stations on what was known as the Watford Branch. These stations were withdrawn from the route in 1982, but would be great to have in TSW as a kind of extra section you can run services at. There was another old branch that is no longer a part of the Bakerloo line, but this branch now exists as part of the Jubilee line, so I don't see it as being needed to be added on to the Bakerloo line in an update of some kind.
You would need to use the LO class 377's for a Watford extension. And you would also have to extend it to London Euston since the LO goes from Watford to Euston
