Passenger seat is and should always be available for err…passengers…want to take the missus abroad…or the kids…or handy for double manning too…I would like the european specs changed, for the drivers rather than always the bosses extra bit of freight. I would also like to see an american style cabin, something on the lines of a motorhome, a shower, a toilet, cooking facilities, and a fridge, tv and definitely a high wattage inverter, a microwave, a bed that turns into a sofa…all of this could be done without too much extra length involved. The cost…well thats what hauliers would love…maybe C/M could ask hauliers if they would be prepared to pay the extra cost, to give their drivers more comfort in their tin boxes away all week !!..a few items could be born by the drivers as they do now, but in my opinion would only add a couple of hundred quid, as the manufacturers would buy them in bulk ( or the boss could ) There would be more people applying to become truckers, there would be a lot more happy drivers on the job. 
the government would have to be involved as changes to the length would need approval, and there would need to be places where drivers could empty the toilet and waste water, and refill the on board water tanks…and i dont think its a lot to ask for…much better than the same size truck being built, all looking the same, with limited spaces.