My suggestions are as follows... Staines West-West Drayton-Uxbridge Vine Street branch lines set in the early 1960's with Class 117, 121 and 128 DMUs. Southall-Brentford Dock branch line set in the early 1960's with Class 117, 121 and 128 DMUs. Slough-Windsor Central branch line set in the early 1960's with Class 117, 120 and 121 DMUs. Maidenhead-High Wycombe/Marlow branch line set in the early 1970's with Class 117 and 121 DMUs. West Ealing-Greenford branch line set in GWE period (2015?) with Class 165/166 DMUs. Twyford-Henley on Thames branch line set in the early 1970's with Class 103, 117 and 121 DMUs. Gerrards Cross-Uxbridge High Street branch line set in the early 1960's with Class 115, 117 and 121 DMUs. Reading-Gatwick Airport (North Downs line) set at any point between the late 1960's and early 2021 with a choice of Classes 101, 119, 165/166, 206 and the soon to launch 769 tri mode (319 conversion) Reading-Basingstoke-Salisbury line set in the 1980's with Class 205 and 207 DEMUs.
I really like GWE and probably will do so again. And I'm all for extensions. So yeah, I'm with you. I especially like the airport route since I always felt it looked empty and wanted to go there when driving by in the past.
Reading Gatwick would be great. Especially considering they are adding the new Class 769 FLEX trains on the route when they come out.