A major issue with the chitern line is pathing. Due to the nature of the route you also need your fast services leaving close together at the beginning of the 30 minutes to allow stopping services to serve more local stations. An example timetable I would give would be:
Xx:00 Birmingham: Haddenham & Thame Pway, Biscester North, Banbury, Leamington Spa, Warwick Pway, Solihull, Moor street, Snow Hill
Xx:05 Oxford: Beaconsfield, High Wycombe, Princes Risborough, Haddenham & Thame Pway, Biscester Village, Oxford Pway, Oxford.
Xx:08 Gerrards Cross: Wembley stadium, Northholt park, South Ruislip, West Ruislip, Denham, Gerrards Cross
Xx:12 Ayelsbury Vale Pway (via Harrow-on-the-hill) normal stopping pattern
Xx:15 Ayelsbury (via HW): West Ruislip, Gerrards Cross, Seer Green and Jordans, Beaconsfield, High Wycombe, Saunderton, Princes Risborough, Ayelsbury.
Xx:25 Stratford upon avon: High Wycombe, Princes Risborough, Haddenham & Thame Pway, Biscester North, Kings Sutton, Banbury, Leamington Spa, Warrick, Warrick Pway, Hatton, Claverdon, Bearly, Wilmcote, stratford upon avon pway, stratford upon avon.
Xx:30 Oxford: High Wycombe, Haddenham & Thame Pway, Biscester Village, Oxford Pway, Oxford.
Xx:40 Birmingham: High Wycombe, Princes Risborough, Haddenham and Thame Pway, Biscester North, Banbury, Leamington Spa, Warrick, Warrick Pway, Lapworth, Dorridge, Solihull, moor street, snow hill (extend to kidderminster at peak)
Xx:43 Ayelsbury Vale Pway (via Harrow-on-the-hill) normal stopping pattern
Xx:47 High Wycombe: Wembley stadium, Sudbury Hill Harrow, Northolt Park, South Ruislip, West Ruislip, Denham, Denham Golf club, Gerrards Cross, Seer Green and Jordans, Beaconsfield, High Wycombe.
In addition to this the Princes Risborough - Ayelsbury branch would run hourly serving all stations.
AND
Oxford to stratford upon avon service calling all stations (replaces sporadic gwr service and leamington stratford shuttle)
This timetable would provide (from London)
Ayelsbury: 3tph
High Wycombe: 6tph
Oxford: 2tph
Banbury: 3tph
Birmingham: 2tph
Stratford: 1tph
Kidderminster: Peak only.
Various station calling patterns could be ammended in the peaks for skip stop service to allow better timetabling or resilience.
This has been easily acheived by:
Binning all banbury and risborough terminators, extend them somewhere useful or free up a path. This timetable would still permit another fast oxford or birmingham service to run if you path it correctly. Spacing is about as good as you get with the 2 track nature of the line.
With improved signalling pathing would be made easier.
I think i recall that headway out of Marylebone is about 2-3 minutes so have factored this in. Trains are timetabled to almost catch up with each other but not quite to maximise capacity.