From my point of view, the ability board trains driven by the AI or other people would be a nice addition. It would be nice to travel along with other friends in the cab and/or in the carriages of the same train, driven by one of them, or by the AI.
That would probably include us to be able to join a server without being assigned a train; just select a station to spawn in, walk around and board the trains that are running around and travel, like some kind of on-foot free roam.
A further improvement from this would be if we could be able to also take over and drive the trains that are driven by the AI or even other people (only while stopped and only if they approve).
All these ideas may also be applicable to the single player mode. Train Sim World already does something similar to this idea, but in single player mode only, as that game has no multi player mode.