@Lenne Maps need a good compromise between a lot of things. They need to be horizontally expansive, somewhat vertically stacked, and easily traversed. Additionally that space needs to be well-filled.
Let's look at some extremes.
TranZit is the most horizontally expansive. Relative to how huge it is, the number of vertical areas you can access is negligible. That map is basically flat. Additionally most of that space is huge and empty. Additionally traversing the map is difficult. And I don't just mean "oh the denizens are annoying." I mean the map is forcing you to go with this circuitous flow. Just the nature of a map that works like a giant clock makes you feel forced to turn with it. It's not like old maps where you just go wherever you want, because back then there was no right way to go. In TranZit there is. Imagine TranZit, but with the entire "middle area" filled in. No fog. You can go off of the bus path and cut across the middle to any other stop very quickly. Now that would giveyou more freedom and kill hassle, but still encourage you to follow a flow (without forcing you to do so.)
Die Rise on the other hand is obviously the most vertical. The size of each floor is nothing relative to the amount of floors. The space is more well-filled, but yet again this is made asinine by how difficult it is to travel around the map. Well, traveling in general isn't exactly difficult, but again the map "tells you" where you're supposed to go. Again you have to go in a kind of circuitous flow. It's just a much more convoluted loop. Down this building, across, down, across, up, up, bam you're back home. Sure you can "cut across" in a few specific locations, but it's hardly free movement. Imagine if instead there were 3 adjacent skyscrapers, each with fully intact stairwells, and sky bridges connecting them each on multiple floors. Still vertical, just less asinine.
MotD was a good combination of horizontal and vertical, in the sense that a given area has enough open space relative to the number of areas. It's still structurally kind of a series of tubes (like the internet) and there are a lot of small between areas relative to the distance between, say, the cell block and the spiral staircase. This is structurally a huge improvement, but still getting from one side of the map to another is difficult, if not forced. Remember back to the days where you'd link the teleporter to the Der Riese mainframe with like 17 seconds left on the timer and how achieved you felt. I think we should be able to make a map larger without changing its ability to be traversed easily.
I won't go too in-depth about buried. It structurally is very similar to MotD in how there are lots of areas, some verticality, and difficulty going from one side of the map to another. (Maze to tunnels? Psh.)
Origins I believe is my absolute favorite in terms of layout. It really has an open layout, allowing you to basically go where you want. It's not confined to a certain path you have to take to get around the map. You kinda just... Go places. It's nice. Not that origins is without problems.
Also no hate on BO2. I was intentionally being overly critical of each map. In fact I'm among the few who loved every single map. I'm just saying in conclusion: the main thing that makes or breaks a map, (in terms of just layout and structure) is how easily and freely you can travel through it. I respect things like the bus for what they are, but on paper it is better than in practice. Zombies has always been kinda sandbox-y. "Here's some core mechanics, do whatever you want with them." In general BO2 felt too on-rails. Not that that makes for a bad experience, but that isn't the Zombies experience. I respect 3arc for experimenting, and the results were conclusive: take a half-step back.