Be able to have other players to join as one company instead of a company each, this would be great in MP.
Have a CB in cab which you can talk to other truckers (MP only of course).
More maps plus other types of roads not just motorways and A roads, this will make it more challenging.
Sat Nav with voice.
I don't know if its possible, but maybe things like realistic cab shapes cause I noticed the default Scania R and Streamline cab aren't the same as their real life counter-part. Might be another copyright issue to "cab design".