Well, first off, the fact that one can't hop onto the back of the truck if it's locked, nor hop out. Should change it so it's always open, maybe have it require an agility check to hop onto the back of a moving truck.
Secondly, people on the outside of the vehicle should be able to see if there are any people sitting on the truck bed and vice versa. People on the truck bed should be able to look at the outside, and speak with people outside, maybe even throw or give items. And they also should be exposed to the weather.
This would make the vehicle in question a perfect candidate for independent mobile distribution enterprises, and inject an element of playful risk as any person would be potentially able to hop in and hitch a ride. The cargo loading functions would have to be tweaked a bit to account for an always-open cargo bay, but I feel like it's a fun tradeoff.
