Easy - Crossrail 3 should be an AC electrified route from both the Amersham and Wycombe routes operated by Chiltern. Express services would route through Old Oak into Paddington; allowing the slower stuff (Aylesbury, Gerrards Cross, Wycombe, Bicester North, Banbury, perhaps also Winslow) to drop down from St Johns Wood / Lords Tunnel underground to Baker Street (Marylebone would be closed), then the trains would stop at Tottenham Court Road, Blackfriars (new platforms on Waterloo & City added), Bank / Monument and rising up out of tunnel near Shadwell.
Fenchurch Street and Marylebone as prime real estate could be re-purposed / built over; and with the conversion of Grays - Upminster and Upminster - Romford into a Tramway system (with a flyover to segregate from other traffic) you could dedicate up to 4tph to the Rainham route, 4tph to Abbey Wood via Barking Riverside and 16tph to Laindon, Basildon, Leigh on Sea, Southend and Shoeburyness. The 4tph from the Gospel Oak to Barking would also operate down towards Grays giving new journey opportunities.
Double-deck stock would be used in 10 carriage, 24 metre formations with wide doors, toilets, First Class etc. all available. Some 5 carriage units would also be used, partnered up in peak where required.
This would make the Chilterns and the Essex Thamesside into prime development land also; and work to give maximum capacity towards a major interchange at Abbey Wood. An underground extension of the Barking Riverside - Abbey Wood line would also include Bexleyheath (station) and an Underground terminus at Bexleyheath Town Centre as the Council there work towards more priority bus routes / trams in the Borough radiating from Bexleyheath.
To achieve a combination of faster and slower commuter services towards Southend, the route would be four-tracked east of Upminster with a 'slow' and 'fast' pair of tracks; the 'slow' pair would be as far as Leigh on Sea with trains via Tilbury also extending this far; the 'fast' pair would enable services from Shoeburyness to run non-stop and more regularly.
Two-tracking of the Stratford-on-Avon route as well as re-modelling at Hatton, Leamington, Princes Risborough and High Wycombe would enable fast services to operate with four-tracking between Moor Street and South Ruislip in any event for the express services from Moor Street to Paddington; creating more paths on the West Coast Main Line.
Indicative peak services [Peak Directions]
2tph Birmingham Moor Street to East London / Essex (Moor Street, Acocks Green, Olton, Solihull, Widney Manor, Dorridge, Warwick Parkway, Warwick, Leamington Spa, Banbury, Bicester North, High Wycombe, Beaconsfield, West Hampstead, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
2tph Oxford to East London / Essex (Oxford, Oxford Parkway, Bicester Village, High Wycombe, Beaconsfield, West Hampstead, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
2tph Coventry to East London / Essex (Coventry, Kenilworth, Leamington Spa, Banbury, Kings Sutton, Bicester North, Haddenham & Thame, Princes Risborough, West Hampstead, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
2tph Stratford on Avon to East London / Essex (Stratford on Avon, Stratford Parkway, Hatton, Warwick Parkway, Warwick, Leamington Spa, Banbury, Bicester North, Haddenham & Thame, Princes Risborough, West Hampstead, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
4tph High Wycombe to East London / Essex (Wycombe, Loudwater, Beaconsfield, Seer Green, Gerrards Cross, Denham, South Ruislip, Sudbury Hill, Wembley Stadium, West Hampstead, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
4tph Gerrards Cross to East London / Essex (Gerrards Cross, Denham Golf Club, Denham, West Ruislip, South Ruislip, Northolt Park, Sudbury Hill, Harrow Road, Wembley Stadium, West Hampstead, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
4tph Aylesbury Vale Parkway to East London / Essex (Aylesbury Vale, Aylesbury, Stoke Mandeville, Wendover, Great Missenden, Amersham, Chalfont, Chorleywood, Rickmansworth, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
4tph Aylesbury to East London / Essex (Aylesbury, Great Missenden, Amersham, Chalfont, Rickmansworth, Moor Park, Harrow on the Hill, Baker Street, Tottenham Court Road, Blackfriars, Bank / Monument, Limehouse, West Ham)
And in the other peak direction...
4tph Chiltern Route to Shoeburyness fast (... West Ham, Benfleet, Leigh on Sea, Chalkwell, Westcliff, Southend Central, Southend East, Thorpe Bay and Shoeburyness.)
4tph Chiltern Route to Shoeburyness semi-fast (... West Ham, Basildon, Pitsea, Benfleet, Leigh on Sea, Chalkwell, Westcliff, Southend Central, Southend East, Thorpe Bay and Shoeburyness.)
4tph Chiltern Route to Leigh on Sea via Rainham (... West Ham, Barking, Dagenham Dock, Rainham, Purfleet, Grays, Tilbury Town, East Tilbury, Stanford le Hope, Pitsea, Benfleet, Leigh on Sea.)
4tph Chiltern Route to Leigh on Sea via Basildon (... West Ham, Barking, Upminster, Laindon, Basildon, Pitsea, Benfleet, Leigh on Sea.)
4tph Chiltern Route to Bexleyheath (... West Ham, Barking, Renwick Road, Barking Riverside, Abbey Wood, Bexleyheath and Bexleyheath Town Centre.)
4tph Chiltern Route to Laindon (... West Ham, Barking, Upminster, West Horndon, Laindon.)
(4tph Gospel Oak to a station such as Grays or Stanford le Hope)
Construction of a major new depot would be completed at Shoeburyness with associated Carriage Sidings; along with stabling at East Ham. Smaller depots would be used at Banbury and Aylesbury with stabling at Tyseley and Hinksey.
Drivers based at Moor Street, Banbury, Oxford, Aylesbury and Wembley Stadium would be used in the West and at Barking and Shoeburyness in the East.
10 car trains could be in a mostly suburban layout; with the order of 5 car trains designed with more improved facilities such as on-board vending machines, First Class (i.e. not on 10 car trains) and more toilets; these used on trains to Oxford, Coventry, Stratford and Birmingham; or to Shoeburyness in the east on premier peak trains.
Grade separation and building would be required at Barking; with more turnback facilities / capacity on the District's eastern end to allow more H&C trains to run through.