I think that every route needs a few extra, so here are some for SEHS. From what I've seen, St Pancras is a bit empty. Perhaps DTG could reskin the HST into an unbranded livery (it could be a replica of the EMR or EMT livery with no logos, or something original like a 'Dovetail Rail' livery or perhaps 'DoveRail') of sorts and have some AI services coming into and out of the MML platforms at St Pancras. Also, I think DTG could slightly modify the Class 377 (and yes, I do mean the 377 and not the 375) to make a 377/5 which can run to Gillingham. All that would need changing is the livery (the 377/5s are, contrary to popular belief, not blue - they are in fact First Capital Connect purple with light blue doors, and unlike the 375s they have a grey roof and a Southern interior, which would only need to be copied from the ECW 377). There's also a freight yard that HS1 runs through. I haven't played the route so I'm not sure if there's anything there, but if there isn't some static wagons and locos would be nice.
I remember them mentioning once on stream that their survey results suggested players would prefer to have no trains, as opposed to fictitious or low quality "filler" trains that exist solely for scenery, and thus DTG don't consider it a worthwhile use of dev time when it could be used to improve other features. I imagine it's probably more immersion breaking to have "filler" trains than to have no trains at all for many players. It's still a valid point though about the emptiness, if they did end up developing appropriate trains for other routes it would be great if they could add scenery services to the non-playable area of other routes later on.
That's weird. For me it's the exact opposite. I would rather have unbranded trains than no trains at all. In TS1 the Chicago racetrack route was one of my favorites, even though it was completely unbranded. And it even had AI only filler stock of other trains to make the route more lively.