I was thinking merely in terms that Marylebone isn't exactly renowned for being the best connected of termini. Moving all of the mainline services to OOC is the obvious starting point.
How is OOC better on that aspect?
OOC is very good for E-W to the City and Docklands but has no N-S variance (I guess, but doubt it would be allowed, that you can board HS2 trains to Euston to have one-change to the Kings Cross area). Marylebone is walkable from Oxford Street (unlike Paddington, let alone OOC), has a N-S tube axis across the West End (so Theatreland, Trafalgar Square, Waterloo, Charing Cross, etc are easily accessible) with cross-platform interchanges to Green Park, Westminster, Victoria, London Bridge, Canary Wharf, and a shortish walk to Baker Street for Kings Cross, Farringdon, Liverpool Street, etc.
I'd argue that OOC is better for commuters (no more walking to Baker Street), but worse for leisure users.
I would be severely tempted to propose moving most if not all of the Oxfords and Bicester North services as well.
So, from Oxford, there's trains to OOC via HW, and trains to OOC via Reading that continue to (poorly-sited, but better connected) Paddington. The 'two different routes to London' concept has gone under your plan.
And from Birmingham/Warwickshire there's either the slow services via HW to OOC, or the super fast ones to Euston via OOC. HS2 will abstract a lot of traffic from Chiltern Mainline services, but this is the way to make it nearly all!
For non-commuter passengers, Marylebone is better than OOC. And Oxfords are not really commuter flows, nor the Mainlines. Bicester North (which isn't a terminus) can have the Banbury semi-fast diverted to OOC for commuters who want that.
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A place to look at for similar is the WAML. Cambridges and Stansteds go to Liverpool Street, the mid-distances go to Stratford (but most stops have Liverpool Street service) and the inners go to Liverpool Street, save for (not yet there) those served by the inner-boosting Stratford service.
2tph mid-distance (Aylesbury via HW, and Banbury) diverted to OOC, replacement 2tph out of Marylebone can either boost the commuter service to places OOC can't reach, or add more leisure service to Oxford/Warwickshire (or both), additional xtph out of OOC to boost commuter/mid-distance service.