as part of this thread: https://forums.dovetailgames.com/threads/gwe-update-for-tsw-2.37378/ we should have the Great western branches these are the: Greenford branch, Windsor branch, Marlow branch and Henley on Thames branch AKA the Regetta line. This would means DTG will need to add the class 165.
This is a repeated suggestion which has many of us queuing up for it, myself included 1,000,000%. However it seems that, according to some, the old TSW2020 GWE route is apparently incapable of being altered to include these branches as well as the 2 that often get left out which are the freight branches to Brentford Waste Terminal and Thorney Mill (Stone traffic)/Colnbrook (Oil traffic) that were once passenger lines but no longer exist. The Brentford line once went beyond the A4 road to Brentford Town and the Dock. The Colnbrook branch once went to Staines West and briefly included a link to the South Western line and going north from West Drayton once went to Uxbridge Vine Street. The Vine Street branch sadly no longer exists in any capacity whatsoever. Lastly, the Bourne End/Marlow branch once went beyond Bourne End and terminated at High Wycombe. A few of the Terminus stations were also once larger than they are today. This is a great suggestion but I fear it will never happen even though I hope it will.
There is also a link line at the west of Old Oak Common depot that runs via Perivale and Greenford to the Chiltern Line via Denham and High Wycombe. This will soon disappear as part of Old Oak Common Station construction as far as I know.
i think i made a suggestion like this but i focused the branch lines on being in Cornwall so you would have the Looe valley line the atlantic coast line the maritime line, the St Ives bay line and finally the tamar valley line. basically you make these into one whole pack for people to play on. i know it's a bit different to your original idea but thought i would give my idea on branch lines