I wonder if Siemens will try and get the contract for Northern Line and Jubilee Line stock replacement when the time comes? Assuming that the NTfL has a successful rollout on the Piccadilly Line which will likely lead to the options for the other lines being exercised then it'd be a no brainer to keep them on for the rest of the Deep Level Tube Lines.
Whilst that could happen long term, the Jubilee and Northern line replacement probably won't be ordered for at least 15 years, with delivery nearer 18-25 years time
I think Bakerloo follow on (to Piccadilly) order is a given (and I wouldn't rule out some of the extra trains for 27tph on Piccadilly being diverted to Bakerloo, so need fewer extra trains)
But still leaves huge gaps between batches (even if Central line replacement happens), so really if Siemens continues to build at Google is going to have to come down to if they can fill the lines in about 5-10 years time (during which might be just some Bakerloo trains, as Central line is looking like stuck until after 2030)
So need to consider what might be built about 2027-2032 (give or take a year), and only realistic UK option is replacing remaining fleets from 1980s to early 1990s. That gives us 15x diesel, 16x turbo fleet, Chiltern 168s, SE London fleet, and some Scottish trains. Out of that should probably be aiming for a regional bi-mode to replace all those diesels either directly or by displacing 170s etc.
Probably a market for about 150-200 5car regional bi-modes replacing all the 158, 159, 165, 166, 168 fleets plus displacing XC 170 fleet, and possibly some of the voyagers. No point chasing the electric fleets, not big enough unless mass fleet renewal in SE London is guaranteed.