I actually think this is a really good idea. Willesden Junction has the potential to be another Stratford/Clapham style hub station, and expanding it seems better value than building a new hub at Old Oak Common nearby. Willesden Junction has several rail routes passing nearby, and would still be well placed to serve new housing at Old Oak.
Firstly, I don't think HS2 needs to stop at Old Oak Common anyway. A better solution would be to just improve connections at Euston to handle the extra passengers - split the Northern Line (increasing capacity on both routes), build Crossrail 2, and integrate Euston Square into Euston. Plus somehow improve the interchange between Euston and St Pancras, giving HS2 passengers direct access to Thameslink. One of the main reasons given for stopping HS2 at Old Oak Common is the connection to Heathrow. But with HS2 freeing up paths into the existing platforms at Euston, could Heathrow Express perhaps divert there? As well as interchange to HS1/HS2/etc, this would give Heathrow Express a more unique purpose to justify its existence alongside the Elizabeth Line.
I also don't see the need to stop GWR services at Old Oak Common. In terms of HS2 connections, the Elizabeth Line would cover local journeys, and for places further west it will often still be quicker or at least easier to use CrossCountry services from Reading/Bristol to Birmingham or beyond than travelling via London. And connecting the Mildmay Line at Old Oak Common is made awkward by having two new stations which don't quite offer direct interchange.
Returning to Willesden Junction, at a minimum they should build new platforms on the semi-fast lines into Euston, allowing services to Tring/Milton Keynes to stop there, maybe other services too. Could even look at upgrades on the Abbey line, including a passing loop, allowing direct trains to Euston. This would improve connections for a lot of journeys, from the WCML to stations towards Richmond/Clapham/Stratford as well as the Bakerloo Line via Paddington - and saving time compared to switching to the Lioness/Bakerloo from further north. This could also address the Southern service to Watford, which provides some useful direct links but is too infrequent. Passengers would instead have several LNR trains per hour from Watford/Harrow to Willesden Junction, to change for frequent Overground services towards Clapham.
With TFL's proposals to create the new West London Orbital overground - which would avoid both Willesden Junction and Old Oak Common, I think the sections could instead be restructured with the Mildmay line to create a simpler network of two lines crossing at Willesden Junction. The Mildmay Line could operate all trains from Stratford to South Acton, then alternating between Richmond and Hounslow. Trains from Clapham Junction would then use the Southern tracks to approach Willesden Junction, serving new platforms parallel with the WCML, then a new curve south of Harlesden connecting to the Dudding Hill line, with trains continuing to Brent Cross West.
Back to the Elizabeth Line, from other comments it seems it would be impractical to use existing tracks to divert via Willesden Junction, but a tunnel could have worked as an alternative. Rather than emerging at Westbourne Park, the tunnel could have continued west to sub-surface platforms at Willesden (perhaps including an extra terminus platform, extending trains from Paddington), with a tunnel portal at the sidings just west of Acton Main Line station. Acton Main Line would have to be closed or relocated in this scenario. The only remaining rail service through this area would then be the Central Line - perhaps new platforms could be built for the Mildmay Line at North Acton, offering interchange to the Central Line, and with a new eastern entrance serving the southern end of the Old Oak Common redevelopments?
