The Rules I am using to modify the game maps are :
Attempt to Minimize changes to match what is visible in the game and indicated on the map for that space.
Simplify the basic primary structure (external shell) which is simpler construction-wise for the conditions the building would exist in. Long flat continuous (thick) walls when possible, instead of fancier concave shapes that are weaker, waste concrete, increase complexity, and lessen internal volumes. All the original map interior spaces have to fit within the simplified borders. The same floor outline will generally be extended downward in its maximum extent (no overhangs) and often upward as well to try to make the external structure monolithic for simplicity.
External walls are to have consistent thickness (with them growing a bit thicker as you go down levels). Basement level is frequently extra thick because it must also anchor and support the entire structure above to the seabed/foundation. Poured Concrete adapts the regular structure to the irregular excavations made into the bedrock.
Internal Walls on different floors are attempted to be superimposed to transfer support strength through the structure.
Internal walls thickened (to help reinforce the overall structure) where it doesn't change what is originally shown (this can be freely done inside all the added areas). There is a little leeway with the wall thicknesses because of the map resolution that I sometimes can widen existing walls a little.
Windows have to be adaquately structurally supported (they are potentially the weakest points in the building design). Windows to be added logically to added interior spaces (try to follow usage pattern of the original level's theme ). Unfortunately many of the external views don't match the seen interior spaces in-game.
The added space within the simpler monolithic external wall shapes is to be used in a logical manner.
Increase the Residences when possible (in appropriate places) - people of Rapture had to live somewhere.
Add new interior sections that are suggested by blocked doors and internal windows -- to fill out volume encompassed by the simplified outer shell. Add doors as necessary to access obvious interior spaces that have no access in the original map (ie- Sirens Alley - a whole block of lit windows, but no door in). Sometimes stairways from another level can be added to allow access. Some added doors are in less obvious places and others at points where the wall is obscured or damaged (where one could have been)
Large boarded up portals that should go someplace - main passages to something significant. Link to another main level area, or to external transportation (ie- an unseen Train station)
Add/increase Basement space for all the boring stuff (utilities, drainage, etc...) which would be needed in a real building. Bulk concrete (with alot of filler rock) is used to fill-in much of that space, with access tunnels for inspection and the network of drains. Water sealing doors added at basement to comparmentalize - uses simpler manual doors, versus fancy 'automatics' up where people are.
Add features that would be logical to facilitate transportation (sub docks/train stations/airlocks). Airlocks are usually at basement level (to be near seebed surface), and there usually are several per level, spaced well apart. Mechanisms to operate automatic airlocks (particularly ones with doors that slide into floor) are located on level below.
Space remaining can be used for assorted utilities and storage. Airlocks and Train systems need nearby space for machinery to pump water/air/reservoirs and to operate the doors. I generally have left the more intricate utilities (pumps/air/heat/power/water/pneumo) system details off (save that fun for the MMORPG).
What cannot really be shown is that in between the floors are fairly thick solid compression plates of reinforced concrete that assist/reinforce the external walls to hold out the massive pressure. Some game level floors are rediculously thin (not really visible to Player), and making them alot thicker (like 4 feet) also wouldn't be too obvious a difference.
I start to get claustrophobic just looking at the interiors the game portrays (would be like spending your life trapped inside The Pirates of the Caribbean Ride at Disneyland) and having more interior space is to be preferred. Hopefully, the MMORPG will be able to have larger interior spaces (because anyone planning this place could visualize how cramped much of what we've seen is, ahead of time and need not be as much a caricature.)