The heavies could do with more parts in general, for now atleast we have daddelzeits bus mods and GTGs truck mod...
generally the devs tend to spead out the vehicles across as many different demographics as possible, mainly because there would be very little point to taking the same amount of effort to make another similar car when that time could be spent making something that doesn't exist in the game at all yet. that being said, I could see them adding some sort of modern european cab-over, like a mercedes/scania/volvo eventually, and the T-series could easily get facelifted in both directions to span from the late 50's (e.g. Peterbilt 351), to the modern day.