I have a few suggestions of my own. It’s likely they came up before so my apologies in advance for that!
-At the moment when people travel between their home and their destination they take the shortest route whichever can get them within 20 minutes. For instance if you have a train connection between towns A – B – C, and a tram connection just between A and B, people WILL travel between A and B by tram, even if the train gets them there faster. Another example is ofcourse when people cause traffic jams on a dirt road, while an empty four-lane highway is running between the same two towns just a few meters more east. Yes I know they partially solved the traffic jam issue by having people look for an alternate path, but why don’t have them go the fastest route to begin with?
-If you have 2 or more lines sharing the same route between 2 or more Railway stations or bus/tram stop, passengers will wait for the trainline with the highest frequency instead of just getting the train/bus/tram which arrives first which too takes them to their destination station/stop. In this case having the ‘same’ line with a shared route with just different destinations (like having a Line 14A and a 14B) is useless because of this.
-If you have multiple vehicles on the same line, they have a short wait when loaded up on passengers before continuing on to the next stop, except for the ‘first’ vehicle in that line. This applies to both road and rail, and it’s probably meant to get some sort of synchonisation in the line so that each vehicle is evenly spaced between eachother, or at least that’s what it should do.
Especially in those years when old vehicles are being replaced by those which are twice as fast like the early 1900’s, my rail lines usually end up with a few 100km/h locomotives lagging behind a not-yet-replaced 50km/h one. When all are finally replaced with similar speed trains I usually have 4 in a row just behind eachother while half of the towns along the line have to wait several months (years) until the first train the row comes around again, taking along most of the passengers waiting, while the other three trains are only filled by 1/4th or something.
To better have those trains (same applies to tram/buses ofcourse, but a bit less since for those relatively short distances the waiting time is sufficient in most cases) synchronise with eachother so they would actually have a 7-minute frequency instead of a 3/3/3/19 one, my suggestion would be to have a selectable waiting time (per line selectable?), where you can specify the maximum waiting time for your vehicles to get in line with eachother (up unto a maximum of 30 real-time seconds?)
It would be a better solution than to impose an artificial frequency by reducing the amount of signals, especially since that’s not really a solution when you have multiple lines running along the same stretch of track. And it doesn’t work for those blue trams lagging behind those slow steam- or Halle trams.
-This has been said many times by now: an option to have your line electrified just by click on a button, instead of manually going through each section and missing those tiny peaces wondering why your e-locs run off in the wrong direction. It’s SO tedious, especially when I already have a sizeable network to begin with, and in those years that the only non-electric loco is the A4 (yes and the railbus…). And also like suggested: make this option available to converting to high-speed tracks as well, and upgrade the signalling along the way.
-Maybe a stupid suggestion, but how about the ability to combine trams to be able to carry more passengers on those highly succesful tramlines? I know the current bus/tram station is already too small for the modern low-flour trams, so it’s just a suggestion.
-HUD annoyances: the fact windows open behind other windows or HUD elements on seemingly random places on the screen is really annoying.
-Have a feedback window instead of those slow popup windows telling you vehicle X is reaching it’s lifetime limit or that you have to connect station Y to the road network or that train Z is now available. But this has been suggested as well.
Other than this I have nothing to add at the moment.