This is only a minor idea, but when selecting a train, a list of all individual units (e.g 313209) there could also be the ability to assign liveries to individual units so only that unit would show up with that livery in timetable mode. This wouldn't add a lot but it would add a bit more personalisation to each run.
This would be really nice. Sometimes I want to make a real world one-of livery, but then it shows up on 5 trains in the same run. I'd love to be able to make my own trainbow-type liveries (as on the Javelin)!