If DTG were willing to create the route from Hayes and Harlington to the Heathrow terminals, I think this would be a really fun add on to add more variety to GWE, and finally have a train that makes use of the wires out of Paddington. Seeing as the route in TSW2 is set in the mid 2010s, the Class 332 would be the best fit, but if they wanted to save time then they could just make some modifications to the 377 model and call it a Class 387, which is currently brand new to the route. I'd also like to see the Heathrow Connect services with the Class 360, or perhaps the new TfL Rail services with the Class 345. I know DTG have said they don't wish to add extensions to existing routes, but it's fun to imagine what could have been if DTG were willing to experiment and spruce up existing routes.
Licencing is the problem. Ferrovial, the owners of Heathrow Airport Ltd, are a hard nut to crack. They rarely if ever allow their brand to be used elsewhere.
That's a shame. I don't suppose they could just re create the livery without the logos? I doubt it's that easy though. I'm guessing TfL rail could perhaps be easier? They already worked with TfL to bring the Bakerloo line to TSW. Or is the fact that trains would be going to stations with "Heathrow" in the title be enough to land them in trouble?
Also note that the class 332’s wore liveries that advertised companies such as Tata Communications and Vodaphone. They would need to get permission to use the logos from these companies and the operator The class 360 would be easier to do if in TFL livery, because dovetail gAmes already have good relations with the company
Also something to remember is that the branch is privately owned by the airport who may not be willing to give access to their stations. Taking the number of reference photos needed without permission would probably attract the attention of airport security and plod (airports are restricted space in UK law)
