I was planning on going with the "powered down base" idea (if that's okay with you) when the player
first walks in, which will mean that in addition to lights and stuff being off a lot of areas will
be inaccesible (though not all), with doors not having power to open and some being stuck halfway
up or down (some of which you could crawl under.) Also if you want I can have noises that will
trigger as the player progresses like something moving through the vents (though this will be
finishing work.) After you turn on the power the lights and door start working again, (as well as
any air intake fans that are visible) and a couple of ambushes happen through vents and some of the
newly openable doors (monsters will be teleported to locations after having seen the player, so
they'll seek out the player from the location they're teleported to out of sight of the player so
it appears that they were aware of the player and only couldn't reach him because the power was
off.

After that there are more rooms available to explore, with the whole layout being non-linear and
without "dummy" doors that are still unopenable. The elevator is unusable, but becomes usable with
either a key, a switch or whatever objective(s) you have in mind. After that the elevator becomes
usable and the player can access that level, which will either be a different part of the same map
or a hub (whichever you prefer.) I don't know what will come after that, but it will be worked on
and mostly finished prior to major detail work so that if sequences are changed it doesn't result
in a lot more work being done.

The way this would go in terms of production would be that I would lay out the rooms with some
detail (like basic texturing and sloping, unless you don't want me to) and basic scripting for
which areas are inaccesible before the power is turned on and what would be required for the
monster amubushes (with modified but still vanilla-ish placeholder monsters--if you want different
behaviors from normal monsterss like faster biting, running or other stuff let me know) as well as any
such stuff for later levels, after which I'll send it to you, you tell me what changes you would
like me to make, then I'll send it back with changes, then you do the detail work you want as well
as any other changes you'd like to make, then send it back and I'll do the changes for the "powered
down base" as well as any other similar such scripting stuff for later levels.

Any step of this is of course at your discretion, so mention any changes to any part you'd like to
see made.