But there is verticality to many of the bases. The village Neiteio mentions being a prime example. OKBzero has it too. You've got the catwalks up top, sewers down below or the main level. Also Mission 22 on the R&D platform is the most vertical space in an MGS game this side of a 100 floor tower in MGS1. All FOBs are extremely vertical in general. What about the village Miller is in? Or the lion king rock, aka the abandoned village? Both very vertical as well. Not only is the whole place up hill, but going vertical first and traversing lower is extremely beneficial in both. Have you explored the bigger areas like OKB zero? filthy with just as many side routes as camp omega. Even the ruins area is vertical in the actual caves. Oh and the lufwa valley mansion is extremely vertical and filled to the brim with various ways in and out.
Like I said, there are a small handful of bases that were actually fun and interesting, but they're few and far between, and none really approaches the complexity of GZ. I liked the design of the MB platforms quite a bit and wish they were worked into the story more than the one FOB tutorial mission. Likewise, I wish they had taken design cues from the MB platforms and created similarly complex structures in Afghanistan or Africa, but that never happens. Having a stealthy sniper battle or fight with some mist skulls or Walker Gear enemies in a location built similarly to the complex MB platforms would have been terrific and forced the player to both move and think in all three dimensions, but no dice. We even see what could have been a great playspace in the area right at the very end of the OKB0 mission but are only allowed to walk along a small portion of it before getting hit with a cutscene.
Even the village you rescue Miller from seems like a missed opportunity. There are only a handful of places where you can get onto the roofs or elevated walkways, but the majority of the rooftops are inaccessible, and you're prevented from making running leaps from one elevated area to the roof of another building. It also has a few interior areas, but by and large they don't really connect in a meaningful or interesting way, or even allow you to change elevations while in the interior (aside from just one building I believe). Compare this to my UC2 Tibet example, where you can dart in and out of interiors, climb internal stairs, pop outside to run across little elevated bridges, drop down behind walls and bits of rubble on the ground, climb the exterior buildings using them simultaneously to aid movement and provide cover, etc. That kind of playspace would have worked brilliantly with MGSV's movement and items, would have been fun to use to the player's advantage, and would have had a ton of opportunities for enemies to show up in unexpected areas to surprise the player if they tried to move too quickly without caution.
And again, those few areas you mentioned along with perhaps 2 or 3 more are basically the only areas that approach interesting design, while the vast majority of the locations presented are basically flattish swaths of terrain (with perhaps a hill or two) and a cluster of solitary, simplistic 1-2 room structures plopped down. Afghanistan had more interesting level design than Africa and provided a couple bases that allowed you to move directly from an elevated terrain position to actually dropping down onto the structures that you wanted to infiltrate. For the most part though, the terrain in the majority of areas either served to funnel you into one of a few different starting choke points, or simply provided a distant overview before you had to drop down to ground level in order to make your actual approach.
I think that's true. But a lot of people are unfortunately wary of doing that, and stick to their preferred method of playing.
Unrelated to the movement mechanics/controls (which I love) or the level design (which I feel was one giant missed opportunity after another), this ties into a gripe I had with the macro aspects of the game's design regarding the research and F2P-esque time wasting mechanics, which was that, despite the game forcing you to repeat areas so frequently, it did its damndest to make it annoying and tedious to acquire new gear in a way that would lend itself to experimentation. There are so many fun, interesting, and useful tools that
can be used, but so many of them are gated off by hours-long waits for MB platforms to upgrade, so you can fill them with the staff you need, so you can start 20 minute to hour+ research times. If you've been investing in one 'primary' playstyle/equipment set and you see a side op that's going to be in a place you've already visited, doing a task you've already done (make a thing disappear), the game makes it hard and annoying to try something completely different on a whim.