For the fun operation of passenger coaches, the roof of the passenger coach needs to have a hinge that allows the top to open up, allowing children and adults to load and unload all of the passengers at each stop.
This is something that Lionel, mth do not have. Even if the roofs just sat snugly or snapped on it would greatly increase the play value.
A fit as simple as the tops of the @Menards delivery vans would do the job.
Yes, I know this means some roofs would get lost by children but you can't win with everything. My kids love putting stuffed animals and little people or other toys inside freight cars to take a ride. Being able to add people or critters to a passenger without having to flip it upside down would be really nice. Kids and many adults I assume, would rather not glue figures to a seat to keep them in place while rolling the carbody back over to screw reinstall the screws that hold the roof in place. The drawback to this approach is that getting the lighting attached is much harder. If you went with an easy open top and lights at the same time another cross brace or something would be needed to hold the lights. It would also potentially mean switching from a two piece to three piece construction.
piece 1 - removable roof
Piece 2 - rectangular car body with cross bracing for the lights and wires, but most of the car top open. molded in steps and vestibules. No opening vestibule doors, prices need kept down. Window strips optional.
piece 3 - frame, either stamped metal or plastic. since an open top capable car would be meant for play value, any electric components for lights would be mounted under the frame.
additionally parts - passenger looking truck assemblies, both with roller pickups. Possible some type of box to house the electronics under the frame. Other than that as few detail parts as possible. Kids break stuff off. For that matter, so do I.
Lionel baby Madison style cars are okay with me, and make sense with lots of road names. I am not sure what length those. Longer cars requiring bigger curves limits the market. I the Madison style car was selected I would avoid rivets just to make paint easier.
Of course other options would be to skip lights on the inside all together, or have conventional bulbs mounted in the floor.
Frankly the idea is exciting to me, because anything that actually makes my kids want to truly play with the trains is an improvement. Also a passenger car I can get for a reasonable price point to custom paint myself is attractive as well. My middle child has a customized "Stephen Express". It used to a set of Lionel MLB themed cars that I got for a song. customizing those cars for him was something special.