I feel like it would make more sense to run some fast services into OOC, rather than Marylebone, allowing the suburban stations to be served easier.
Ideally, it'd like to see
Full Electrification of Chiltern, EWR and Snow Hills
4 tracking Dorridge to Moor Street
Reinstate loops where it makes sense
Rough fantasy timetable:
2tph OOC - Birmingham Moor Street: High Wycombe, Haddenham (1tph), Bicester N, King's Sutton (1tph), Banbury, Leamington, Warwick Pwky, Dorridge, Solihull
2tph London Marylebone - Oxford: Gerrards Cross, Beaconsfield, High Wycombe, Princes R (1tph), Haddenham (1tph), Bicester Village, Islip (1tph), Oxford Pkwy
1tph OOC - Stratford-u-A: West Ruslip, High Wycombe, Princes Risborough, Bicester N, Banbury, Leamington, Warwick, Warwick Pkwy, Hatton, etc. I would even consider making this into a splitter at Leamington, combined with the Nuneaton service.
1tph Moor Street - Stratford fast: Solihull, Dorridge, Lapworth, Stratford Pwky
1tph Moor Street - Oxford: Solihull, Warwick, Leamington, Banbury, King's Sutton, Heyford, Tackley
2tph London Marylebone - MKC via Amersham
1tph London Marylebone - Aylesbury via Wycombe stopper
1-2tph London Marylebone - High Wycombe via Denham stopper
4tph Dorridge - Stourbridge (2tph)/Kidderminster (2tph)
4tph Stratford (2tph)/Whitlocks (2tph) - Stourbridge (2tph)/Worcester(2tph)