There are many options on the cards, the present 'Boundaries' being dictated by where major new works are needed.
Beyond Shenfield, Abbey Wood and Maidenhead, works can be covered ither by T&W Act applications, or if for railway purposes and within the existing curtilage of the railway, then just local applications for the building works are required.
So Service extensions beyond Shenfield towards Southend are possible, say 3 per hour between existing Semi-Fasts which run as now, so improving connectivity between Southend branch stations and the Shenfield services operating as now.
Services beyond Abbey Wood could be extended at least to Dartford, again every 20 minutes or so, Gravesend or Ebbsfleet Interchange are possibilities.
Services beyond Maidenhead to Reading have been designed for in the Reading rebuild, and extending up to 10 tph beyond Old Oak Common to some other destination is inevitable, WCML to Tring and Milton Keynes is logical, but no further, but railway operations around these West London nodes is still in the melting pot.
Food for thought?
Benefits, better in-town penetration and distribution, reducing the need for interchange at the edge of centre nodes forced on us by the Victorian landowners 150+ years ago.