If Lionel, MTH, Atlas, etc. produced their trains on the basis of our demand, I would expect a great deal more variety of trains on the market. However, Lionel, MTH, Atlas, etc. produce what they think they can sell the best, so the variety of trains for us is limited to major railroad equipment only. For example, this is why there are so many locomotives of the Pennsy prototype.
I recognize that the variety of O-gauge trains is much larger today then it ever was, so I am not complaining, just theorizing. Eventually, I would like to see the variety of O-gauge trains equal that of HO trains.
One aspect of O-gauge trains I would like to see much more of are undecorated items coincident with an easy and economical way to apply road names to trains other than decals.