OK for this map consider; when you first walk through its pre disaster (the part your making) and then later you will come back to it when its all gone to hell so make it peaceful with a few scientists (not too many) and a barney ot two milling around but make it so that it can be... "forged" into a good combat or puzzel based map later on.
Have some high tech and neat looking stuff to show off your skill, the player starts in an elevator, get out and makes his way to a Level 7 security clearance point (check the sign textures in the HL wad). The level 7 checkpoint should be at the end of the map, have it so that there are clearance levels 5-3 checkpoints allong the way.
At the end of the map the player reaches a big door (one of the double edged metal ones) This will be like an airlock chamber for level transition. You should include about 4 points where the player has to wait 10 seconds for clearance (use the 3-5 checkpoints), this is where the credits will show up so you can have some interesting stuff happening around the area to keep the player occupied. And there you go...
Heh, if that don't give you the basis for a map I don't know what does.
Thanks for doing this too guys.