I'm happy to see that older preserved routes aren't just been abandoned and DTG is publishing new loco DLC for them, and there's now the preserved crew dedicated to improving them. The improvement I would most like to see on these routes is to add more gameplay (i.e. layers) when a new relevant loco is added (either via a new route or new loco DLC). As an example the 403 was added in HMA, which also serves HRR and RRO. Going back and adding a new layer to the timetable with ICE services would really increase the value of the newer HMA DLC as it would add more gameplay to some of the routes I already have. And yes you can sort-of manually do this already with scenario planner (and I have), but with the tool available you just can't achieve the same level of quality as the devs can through the timetable.
403 can also be seen on MSB, although in reality, it diverts to a Schnellfahrstrecke just behind Lohr. I think nobody would mind ICEs making an "unrealistic detour" to Gemünden. Br 101 is in production, that is an absolute backbone of ICs in Germany, majority of routes need that one. So yes, new layers need to appear.
BR 403 on HRR & RRO BR 420 on RRO, HRR, SKA BR 423 on SKA, RRO The other routes should definitely get the layers. It’ll (most likely) add more services and diversity to them.
One thing they can’t do is keep adding layer upon layer into routes. There isn’t the processing power to run all the layers or the time to program and test new layers in old routes as new locos are added. The most you are likely to get is suitable substitutions of trains into already existing services. New layer capacity in an old route will really only be reserved for new loco DLC for that route and even then they are limited by the performance capabilities. This is the case with HRR. A new loco DLC is coming to that route. There isn’t the (software) capacity for layers from anywhere else. The route would just cease to run. With the newer routes they are likely to be fairly full when released using previous stock for extra layers, and in HMA even that was already too much content for the route to handle. That may also happen to the old routes getting new loco DLC. We’ll have to wait and see.
I'd question how well it would be able to handle the timetable on the route, it's difficult enough with the Class 40 and it's considerably more powerful. I'd imagine you'd need new services to use it, not just a simple substitution, but could be wrong.