Here is my idea, now this is contigent on plans on other parts of the network and some infrastructure improvements to enable it.
London Paddington - Old Oak Common - Reading - Swindon - Chippenham - Bath Spa - Bristol Temple Meads - Weston-super-Mare. 1tph, GWR, 9 Car Class 800.
Increased from every 2 hours to every hour, to give a good fast service from Bristol to Weston-super-Mare.
London Paddington - Old Oak Common - Reading - Didcot Parkway - Swindon - Chippenham - Bath Spa - Bristol Temple Meads. 1tph, GWR, 9 Car Class 800.
Current service, as is, no changes.
London Paddington - Old Oak Common - Swindon - Bristol Parkway, Split 1 Bristol Temple Meads, Split 2 Newport - Cardiff Central. 2tph, GWR, 2 x 5 Car Class 800.
A reintroduction of the super fast services from London, but splitting at Bristol Parkway to save paths in/out of London.
London Paddington - Old Oak Common - Reading - Swindon - Bristol Parkway - Newport - Cardiff Parkway - Cardiff Central - Bridgend - Port Talbot Parkway - Neath - Swansea. 1tph, GWR, 9 Car Class 800.
Current service, as is, no changes.
London Paddington - Old Oak Common - Reading - Swindon - Bristol Parkway - Newport - Cardiff Parkway - Cardiff Central - Bridgend - Port Talbot Parkway - Neath - Llanelli - Carmarthen. 1tph, GWR, 9 Car Class 800.
The current Cardiff terminating service but extended to Carmarthern, this is instead of the new Lumo service, this would better serve South Wales and give better links to/from West Wales.
Manchester Piccadilly - Stockport - Macclesfield - Stoke-on-Trent - Stafford - Wolverhampton - Birmingham New Street - Worcestershire Parkway - Cheltenham Spa - Bristol Parkway - Bristol Temple Meads - Taunton - Tiverton Parkway - Exeter St David's, Extension 2hrly Dawlish - Newton Abbot - Torquay - Paignton. 1tph, XC, 1 X 7 Car Class 8XX.
The current Manchester to Bristol service, extended to Exeter with 2 hourly extensions to Paignton, Paignton sees a lot of demand from Bristol. Also some better rolling stock for XC.
Newcastle - Durham - Darlington - York - Doncaster - Derby - Burton-on-Trent - Birmingham New Street - Worcestershire Parkway - Cheltenham Spa - Bristol Parkway - Bristol Temple Meads - Taunton - Tiverton Parkway - Exeter St David's, Extension 2hrly Dawlish - Newton Abbot - Totnes - Ivybridge - Plymouth. 1tph, XC, 1 X 7 Car Class 8XX.
The current service, but focusing on the core of the route, as customers can change at Exeter for services to all of the South West and better rolling stock too.
Cardiff Central - Cardiff Parkway - Newport - Filton Abbey Wood - Bristol Temple Meads - Bath Spa - Bradford-on-Avon - Trowbridge - Westbury - Warminster - Wilton Parkway - Salisbury - Romsey - Southampton Central. 1tph, GWR, 1 X 5 Car ClassXXX.
As the current service but stopped at Southampton Central, as better connections can be provided from there, with an EMU express service from Southampton to Portsmouth.
Cardiff Central - Cardiff Parkway - Newport - Filton Abbey Wood - Bristol Temple Meads - Weston-super-Mare - Highbridge & Burnham - Bridgwater - Taunton - Wellington - Tiverton Parkway - Cullompton - Exeter St David's. 1tph, GWR, 1 X 5 Car ClassXXX.
As the current service but with a more clockface timetable than currently, with stops removed to speed up the service, and local stops moved onto stopping services.
Worcester Foregate Street - Worcester Shrub Hill - Ashchurch for Tewkesbury - Cheltenham Spa - Cam & Dursley - Bristol Parkway - Filton Abbey Wood - Bristol Temple Meads - Bath Spa - Bradford-on-Avon - Trowbridge - Westbury - Warminster - Wilton Parkway - Salisbury - Romsey - Southampton Central. 1tph, GWR, 1 X 5 Car ClassXXX.
The current service between Worcester and Bristol but with stops removed to make this service semi-fast, intermediate stops moved to other services, also providing a 2nd semi-fast service between Bristol and Southampton, to connect with a new fast service to/from Brighton.
Worcester Foregate Street - Worcester Shrub Hill - Ashchurch for Tewkesbury - Cheltenham Spa - Cam & Dursley - Bristol Parkway - Filton Abbey Wood - Bristol Temple Meads - Weston-super-Mare - Highbridge & Burnham - Bridgwater - Taunton - Wellington - Tiverton Parkway - Cullompton - Exeter St David's. 1tph, GWR, 1 X 5 Car ClassXXX.
An additional service to/from Worcester, and an additional service between Bristol and Exeter. To provide intermediate stations such as Ashchurch and Bridgwater with a half hour service without slowing down Intercity services.
Bristol Temple Meads - Bath Spa - Bradford-on-Avon - Trowbridge - Westbury - Frome - Bruton - Castle Cary - Yeovil Pen Mill - Thornford - Yetminster - Chetnole - Maiden Newton - Dorchester West - Upwey - Weymouth. 1tph, GWR, 1 X 5 Car ClassXXX.
The current service, but speeded up with local calls in and around Bristol and Bath removed and frequency improved to finally provide an hourly service.
Bristol Temple Meads - Bath Spa - Chippenham - Swindon - Wantage & Gove - Oxford. 1tph, GWR, 1 X 5 Car ClassXXX.
As the "new" service to be "re"introduced soon.
Cardiff Central - Cardiff East - Cardiff Parkway - Newport West - Newport - Somerton - Llanwern - Magor & Undy - Severn Tunnel Junction - Pilning - Patchway - Filton Abbey Wood - Lockleaze - Ashley Down - Stapleton Road - Lawrence Hill - Bristol Temple Meads. 2tph, Transport for Wales, 3 Car Class 197.
This would take over stops at Pilning (Have a further idea for how this would be served) and Patchway to allow speeded up GWR services from Cardiff to Bristol (See Below)
Gloucester - Cam & Dursley - Charfield - Yate - Bristol Parkway - Filton Abbey Wood - Lockleaze - Ashley Down - Stapleton Road - Lawrence Hill - Bristol Temple Meads. 2tph, GWR, 3 Car ClassXXX.
Effectively a metro service between Bristol and Gloucester. There isn't a need for additional services between Gloucester and Cheltenham Spa. With other planned services, and changes can be made at Bristol Parkway or Cheltenham Spa.
Weston-super-Mare - Weston Milton - Worle - Yatton - Nailsea & Backwell - Long Ashton - Parson Street - Bedminster - Bristol Temple Meads - Lawrence Hill - Stapleton Road - Montpelier - Redland - Clifton Down - Sea Mills - Shirehampton - Portway Park & Ride - Avonmouth Extension hrly St Andrews Road - Severn Beach. 2tph, GWR, 3 Car Class XXX.
Service every 30 mins, with an hourly extension to Severn Beach.
Portishead - Portbury - Pill - Ashton Gate - Parson Street - Bedminster - Bristol Temple Meads - Lawrence Hill - Stapleton Road - Ashley Down - Lockleaze - Filton Abbey Wood - Bristol Brabazon - Henbury. 1tph, GWR, 3 Car ClassXXX.
The current Bristol Temple Meads to Filton Abbey Wood service, as it is proposed to extend it to Henbury and Portishead respectively once the stations are opened.
Portishead - Portbury - Pill - Ashton Gate - Parson Street - Bedminster - Bristol Temple Meads - St Annes Park - Keynsham - Saltford - Oldfield Park - Bath Spa - Corsham - Chippenham - Royal Wotton Bassett - Swindon. 1tph, GWR, 3 Car ClassXXX.
A second train per hour to/from Portishead to Swindon, via a new station at Corsham.
Severn Beach - Henbury - Bristol Brabazon - Filton Abbey Wood - Lockleaze - Ashley Down - Stapleton Road - Lawrence Hill - Bristol Temple Meads - St Annes Park - Keynsham - Saltford - Oldfield Park - Bath Spa - Freshford - Avoncliff - Bradford-on-Avon - Trowbridge - Westbury. 1tph, GWR, 3 Car ClassXXX.
A new service from Severn Beach via the Henbury loop, Bristol Temple Meads and onwards to Westbury, to provide better links to the new arena.
Other services
Swindon - Royal Wootton Bassett - Chippenham - Melksham - Trowbridge - Westbury - Frome - Bruton - Castle Cary - Somerton - Taunton
Swindon - Royal Wootton Bassett - Chippenham - Melksham - Trowbridge - Westbury - Dilton Marsh - Warminster - Wilton Parkway - Salisbury