A couple of my local lines:
Brighton Mainline:
Grade separation at Windmill Bridge, Keymer, Stoats Nest Junctions
Four tracks extended from Balcombe Tunnel Junction to Keymer Junction
Upgrade sections of the line to 100mph (I believe some areas around Gatwick used to be 100mph to 319s in the past IIRC)
Extra platforms at Gatwick Airport, East Croydon and Clapham Junction, particularly for London-Bound services
Remodelling/rebuilding of Victoria station to remove the horrific dingy low roof and shopping centre above platforms 8-19 and provide more platforms
Abolish Gatwick Express and use the paths to provide proper intercity services from Victoria to coastal destinations such as Eastbourne, Worthing, Portsmouth and Hastings and end splitting and joining at Haywards Heath. Reintroduce the Brighton express (Brighton-EC-Clapham Junction-Victoria) at the same time.
Oxted line:
Redouble and electrify Hurst Green to Uckfield
Extend Uckfield line to Lewes, with a spur to Brighton (similar to the
BML2 Sussex Phase proposal)
Extend East Grinstead line to Haywards Heath, with a new station at Ardingly (sorry Bluebell railway)
Upgrade line between Eridge and Tunbridge Wells, so a Brighton/Eastbourne to Tunbridge Wells service can operate (sorry Spa Valley Railway)
East Coastway:
Electrify Marshlink. Link it to HS1 with the appropriate remodelling at Ashford, and reinstate fast direct Brighton services.
West Coastway:
Re signalling and installation of new crossovers at Worthing station to allow slow westbound services to be overtaken by faster ones. Thus, introduce a regular fast service from Brighton to Bournemouth/Weymouth calling at principal stations only. (Hove, Shoreham, Worthing, Barnham, Chichester, Havant, Cosham, Fareham, Southampton, Brockenhurst, Bournemouth)
Increase frequency of direct GWR Bristol and Cardiff services, using third rail capable bi-modes (769s refurbished to an intercity standard?)
Rebuild the line from Fareham to Gosport