In the long run, the compromise height platforms shared by tube-gauge and mainline-gauge trains need to be resolved one way or t'other.
What to do about Kensal Green to Harrow and Wealdstone?
Terminate the Bakerloo at Queens Park? Not ideal.
Terminate the DC service at Queens Park and extend the Bakerloo to Watford Junction? Not ideal.
Shut the DC line from Euston to Queens Park and extend the Bakerloo to Watford Junction? Not ideal.
Rebore the Bakerloo to mainline gauge? No.
Run tube gauge trains on the DC line? Hmmm. Apparently a Bakerloo train only holds about half as many people as an Overground train, so an increase in frequency would probably be needed, but if it were to be run by LU and subject to LU reliability metrics, you probably wouldn't need any more platforms at Euston.
How about, after the Northern Line has been split in two, giving one of the resulting lines a (tube gauge) branch to Watford Junction? The cheapest option would probably be to dig a pair of slopes under the HS2 station to connect from the City branch to the DC line. Interchanges would be better if the connection was north of Camden Town, but connecting to the Edgware / Charing Cross line would give northwest London two routes to the West End and none to the City, so you would probably want to cross that line to connect to the Barnet / Bank line instead, and I'm not convinced the extra expenditure to serve Camden would be worth it. But the connection under the HS2 station seems pretty cheap, specially since the area has been demolished and the HS2 station is currently on pause.
For completeness I mention that the Croxley Link being mainline gauge would raise compromise height issues unless the stopping service from Wembley Park to Croxley and Watford Junction were made part of the Jubilee Line...
What to do about Kensal Green to Harrow and Wealdstone?
Terminate the Bakerloo at Queens Park? Not ideal.
Terminate the DC service at Queens Park and extend the Bakerloo to Watford Junction? Not ideal.
Shut the DC line from Euston to Queens Park and extend the Bakerloo to Watford Junction? Not ideal.
Rebore the Bakerloo to mainline gauge? No.
Run tube gauge trains on the DC line? Hmmm. Apparently a Bakerloo train only holds about half as many people as an Overground train, so an increase in frequency would probably be needed, but if it were to be run by LU and subject to LU reliability metrics, you probably wouldn't need any more platforms at Euston.
How about, after the Northern Line has been split in two, giving one of the resulting lines a (tube gauge) branch to Watford Junction? The cheapest option would probably be to dig a pair of slopes under the HS2 station to connect from the City branch to the DC line. Interchanges would be better if the connection was north of Camden Town, but connecting to the Edgware / Charing Cross line would give northwest London two routes to the West End and none to the City, so you would probably want to cross that line to connect to the Barnet / Bank line instead, and I'm not convinced the extra expenditure to serve Camden would be worth it. But the connection under the HS2 station seems pretty cheap, specially since the area has been demolished and the HS2 station is currently on pause.
For completeness I mention that the Croxley Link being mainline gauge would raise compromise height issues unless the stopping service from Wembley Park to Croxley and Watford Junction were made part of the Jubilee Line...