Once the Bakerloo line is finished, it isn't a piece of content to be dismissed. Some really good extension DLC can come from it since a lot of the north of the line is shared with the Overground. Eventually you can sample more and more of the TFL network since DTG has formed some sort of a lincensing relationship with TFL. If any Overground extension DLC is to become of the Bakerloo line DLC, it'd best start from Willesden Junction for viability. Some of the ECW Electrostar content and recordings can be reused. Having 2 mini routes built into one DLC would really create a strong feel of TFL's interdependency of their lines.
I think they should add the WCML as a DLC. This would go from London Euston up to Watford Junction or Milton Keynes. This would then allow for stopping Overground, Southern or LNWR services and then fast Avanti West Coast / Caledonian Sleeper services in the area.
They could/should add the line to Euston and up to Watford Junction. I doubt the WCML would be included however (as it's not exactly a long stretch of line,).
If you want to see my suggestion on the WCML from Euston to Milton Keynes: https://forums.dovetailgames.com/threads/west-coast-main-line.23650/