I've posed a CJ-style drop-down tailgate in the tailgate opening:
I have a window that's the right size to install in the rear door.
A side-swing door above a drop-down tailgate would look like this:
Or I could do a one-piece swinging door:
I'm thinking I'll probably do the upper half door and leave the drop-down tailgate on the trailer. That way when I take the topper off to use the trailer as an open cargo trailer I won't have to swap a tailgate back on, I can just leave the tailgate in place for all configurations.
If my primary use for the trailer with topper was as a camper I'd probably do the full door.
I drew the doors in the above images with piano hinges, but on second thought I think the door should swing on lift-off hinges like Jeep doors have. That way the door can be removed if the trailer is needed to carry something that has to hang out of the back.