A completely different idea, what if the Elizabeth Line takes over the Central Line between North Acton and West Ruislip? Chiltern services could all stop at one of the Ruislip stations to offer interchange.
The Central Line could have a curve after East Acton to a new OOC terminus alongside North Pole, and the loss of Ruislip depot shouldn't pose too many issues as a lot less trains would be required on the line.
1.) Where else do you run these surplus Central Line trains? Ealing Broadway? Would be a massive increase in capacity there that would largely remain empty. You can't cut service in the core as the Central Line is still busy there. No room for a tube station by North Pole (the depot is rammed with GWR units) and Old Oak Common station will primarily be the other side of the tracks.
2) You slow down Chiltern even more and don't really provide any meaningful capacity upgrades once the withdrawn 12tph Central Line services from Northolt or W. Ruislip are taken away.
I think it's a decent idea for a cheap option if you absolutely had to do something with the Paddington terminators on Crossrail, but it wouldn't pass a cost benefit analysis and justify the investment.
It's only worth extending the Elizabeth Line up towards Ruislip if it heads as far as Gerrards Cross at least.
Additional options include a EL extension via new grade separated junctions around Scrubs Lane and Northwick Park to Aylesbury Vale Parkway, to replace and improve frequency of services, compared to the Chiltern services currently heading into Marylebone. This option can only be implemented post-HS2 Phase 1 completion though, as it needs paths on the WCML to be made available.
