Pods
Elon originally propose passenger pods that could handle people in a 2x14 configuration a freight/car pod that can handle cars or freight.
Since we are designing the larger passenger+car design, we have more width to accommodate more passengers, so I propose a 3x14 seat configuration (with 2 seats on one side and 1 seat on the other and an aisle between them).
The interior cabin of the passenger pods should have a lavatory and snack/drink machines in the back for use by the passengers during transport.
The seat width and pitch are designed to be similar to first class airline seats.
The standard first class seat pitches and widths are 38" and 21" (respectively, source), so a row of 14 seats is 44ft long and 3 wide is 5.3ft and considering the Elon describes the pod as being about 7.3ft wide, that leaves 2ft for an aisle to the back.
Elon proposed vertical/wing doors on the pods, however, if there were an emergency evacuation of the pod while still in the tube, these doors would not be able to open. So I propose using sliding doors on the pods.
Inwardly opening doors are used in aircraft to cope with the high pressure difference between cabin and the outside. For safety reasons, this should be used for hyperloop.
These will be fairly stock/metal, since they will just have cars. I do think that maybe having places for hooks on the floor for securing cargo/cars will be a good idea.
Also just sliding doors
These are tricky since loading cars from the side would be tough, so I propose a swing back door design that allows cars to be driven straight in the back (and then backed out when they reach their destination). Also, there would still need to be side doors (vertical hinge or sliding, doesn't matter) so the driver of the car can get out after driving the car into the pod.