DD,
I had thought about something similar. Also taking a plasma cutter to the back of a cab and opening it up. Then using a gasket such as they use when attaching a van body to a box truck bed for fleet vehicles. Thus having a walk through pop up camper.
I always thought that'd be cool too but at that point you're really dedicating the rig to being a camper only unless you figure out a good removable door for the cutout on the truck.
Never said I was building it
I thought about using a wider door but in my drawings I was losing a fair amount of cabinet space with the 34 inch door I had planned to use. I looked again at the FWC and concluded that the 28 inch FWC door feels a lot smaller than it is because the bench seat protrudes so far into the entrance. I think that if the door were unobstructed it would be plenty big at 28 inches.
I suspected you weren't building based on your lack of interest (perceived on my part) towards in depth building projects in the past but I previously didn't say it as to not take any wind out of your sails if that was the case.
Going on the limb should I'm assume we're talking sliding wall hard side campers like XP and alaskan for the doorway which would be full height (that is what I'm looking at myself)? If so I was thinking 24" on the lower would likely be plenty, esp. without any obstructions like a couch in the way. The upper would need to be wider to work the hinge layout anyways which is where you could have some more width to accommodate the upper body. But if you got the room for the 28" roll with it.