The point of an interchange isn't really to provide additional services for the area the station is located in, it's to provide new journey opportunities by changing at that station. That's why it's - umm - called an interchange.
The obvious way to implement an interchange here is by moving Wembley Central to the other side of Wembley High Road, so - no additional station on the Bakerloo line. As I recall, the existing Wembley Central station is not exactly the nicest of stations anyway, so rebuilding could be quite welcome. There would be an additional station on the Chiltern line, but since it would be next to a medium sized town centre (which the line currently runs past without stopping), that's arguably a good thing.
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Fair point about the savings being marginal. However missing out the entire Woolwich line is a pretty huge omission (the Woolwich line trains via Lewisham don't stop at New Cross btw so no interchange from them either). Also, New Cross only gets an Overground train every 15 minutes. A station where I suggested would see a much more frequent Overground service, giving massively better connection opportunities. (As well as allowing travel from the SouthEastern area to the Norwood Junction line without going into London Bridge).