One thing I’d love to see is all the rolling stock in game to be updated to include the liveries of all the licenses DTG currently hold. I understand that this would be a big chunk of work but it’d certainly be something that I’d appreciate, immersion is the main thing I look for in the game and keeps me away from routes like Glossop and BCC
I think the major issue here and the likely reason it hasn't been done yet is that a different operator is often more than just a different livery. As highlighted by the Rivet DLC announced, it's often different interiors, different seating layouts, different lengths, different headlights, different cab setups, sometimes even different engines with the EMT vs GWR HSTs. If it's a train not already on the route, it also means creating a new timetable which is a lot of work as well.