Could you make an example? (So I know what to avoid when making terrains 
).
I'm always a bit struggling with making terrains interesting while keeping them simple.
edit:
just noticed that this is a super derail of the topic, maybe it should be discussed elsewhere.
With the ability to script this, I could see step ladders as deployable equipment, light switches inside buildings, secret doorways in bases ect.
These 3 ideas can be used to show my reasoning:
Step ladders - for the player to need to bring those, the map would be required to:
1) Contain a section that could only be reached through the use of those ladders, which then forces the player to always bring one, otherwise it can't get to that section and win the mission or get whatever is in that section; when the player learns about this and always brings one, then this simply becomes another chore and stops being a challenge/novelty
2) Be designed in a way where there's only one access point to the special section, but in that case the player still can move through this bottleneck, since the AI is too dumb to understand the importance of such an access point and defend it accordingly, like a human opponent would. The ladder could still be used as an alternate way to reach the special section, but it will require more time spent by the player in clicking, etc., for no special gain since outflanking will not give any advantages while fighting.
Secret passages:
3) If they are secret, then the player will need to spend time discovering them, otherwise there's nothing secret about them, instead of fighting. And when they are discovered the first time, then they definitely stop being secret.
4) Since they provide alternate access to other map sections, then we go back to the whole bottleneck dilemma - tactically the player gains little or nothing for the amount of extra actions required to use those passages; a bottleneck's role is to funnel both the player's and the AIs units to a single location to maximize and speed up fighting, but if you're adding those alternate passages, then the bottleneck loses its importance, specially if it is designed to serve as a gateway between the player and an objective that needs to be destroyed
Light switches
5) If the map starts completely in the dark, then the player will bring electro-flares from the start once it learns this; so a global switch needs to be placed in a location that can't be close to the starting area (otherwise just design the map all lit up) and if you place it in an objective area that you need to reach to win, then once you reach it there's no point in turning on the lights
6) If the mission is a 'kill them all' then it would make more sense to have a global light switch placed at the other end of the map, but there's already a 'bug hunt' mode in OXCE to speed up finding that last alien, which is more helpful than turning on all the lights
(plus there's the turn 20 limit when the AI becomes aware of all your units' position)
Now, regarding specific terrain design, as you asked, consider the original Terror Site terrain: how often do you usually move units upstairs on the large buildings or spend time checking all the rooms?
The answer is that usually players don't (and this is something I've seen being mentioned in forums about the newer XComs as well, where players tend to avoid getting inside buildings) for a number of reasons:
* AI units will eventually move out of those locations, if they start there
* It is faster to clear a map if you stick to the ground/outside, for both moving and line of sight/fire proposes
* Even better, blow up the buildings from a distance to reveal the interior instead of risking reaction fire (or Chryssalids!)
* A map designer can always spawn enemy units into closets/toilets/etc. like TFTD did in the Ship maps, to force the player to clear every building/floor (and make the walls indestructible), but nearly everyone dislikes those TFTD maps precisely for this reason
However, there's a bit of a sweet spot here, that can be shown through how UFOs/Alien Bases are designed:
* UFOs are essentially bottlenecks that the player needs to move through, in order to kill/capture the last remaining AI units (whose spawn points are designed to keep them in those locations) or to reach the Elerium; same applies to the Command section of alien bases and the engineering block that contain UFO Power Sources in those bases
* You can also design buildings or underground base sections using the bottleneck principle, but if you fill a map with too many of those structures then you risk missions becoming an endless chore (like TFTD) of clearing everything from AI units; this can be mitigated if you give the player some 'reward' for doing so (money, equipment, etc.)
