I thought about this when I had the panels out last month to run my power cables. Really, the engineers did a pretty good job utilizing the space available around the wheel wells, there isn't much being wasted with the panels. Sure, you can remove them and make something that fits a bit tighter, but it's going to be an awful lot of work for not very much in return in my opinion. Best bet if you really want to go that route is cutting out the existing cubbies and fabricating some storage boxes or shelves to go in their place, but good luck making it look good and not be a pain to install. If you really want to squeeze as much space as you can out of the back, then go for it, but it's going to be quite a bit of a slog and require a lot of fabrication to make it work well.
Also, removing the panels entirely will only net you maybe two extra inches in width total, if that. The biggest downfall to the current setup is the way the tops of the bottom trim pieces are angled towards the windows, I think if they had made them straight up and squared off it would have been better storage. Although with them angled like that you can still kind of get into them even with something in the rear.
The rear door on the other hand is a different matter, the access door is a bit small in comparison to how large the opening in the sheet metal is. I think that could be enlarged to make utilizing it a bit better for storage, or just ditching the bottom section entirely to gain a couple inches or so depth wise in the trunk area and make a flat aluminum (or even HDPE plastic) panel to clean up the looks that mounts flush to the sheetmetal. Something to think about with the rear door though is if your vent loses its flaps you will end up with a ton of dust in there. My red H3 was like that and it was extremely dusty in there, and that's without doing a lot of trails.
One way to increase useable storage in the rear that I've thought about is fabricating a thin and wide lightweight storage drawer that is bolted up to the ceiling. The rear door opening actually is a few inches or so lower than the roof and you could easily put something up there that wouldn't chew up too much useable space. Ideally it would not only drop down but also slide out. I think it'd be a great place to stow weaponry and other thin vitals that you need to be able to access no matter what you have in the rear of the vehicle. Hard to describe how I am picturing it. Kind of like a thin wide drawer that hinges at the rear and slides out and then when it's fully out it hinges all the way down so that it's perpendicular to the ground, more or less. With it being up in the ceiling it's out of sight, especially since no one is really going to be looking up for anything, and if they do see it they won't know what is in it. You'd want to keep the drawer itself and anything you store in it fairly light since it is pretty high up, but it would be doable I think. Plus it'd look pretty cool, and we all know that is one of the more important factors to consider.
Biggest trick is figuring out how exactly to mount it securely. I have some rough ideas after building the brackets for my power panel, but I know I'd need more attachment points for a drawer up there.