I'm going to bet that this has probably been thought of and perhaps even already developed internally... but I felt it was worth a mention just in case. It's one of those simple ideas that can get easily overlooked.
The title is pretty self-explanatory really. In Goldeneye, when reached a low ceiling/vent entrance you would automatically crouch. I believe it was originally done by detecting the ceiling height near the player. The most memorable example would be the vents in complex.
As far as I'm aware it's very simple to code a brush entity that forces the player to crouch. It's nothing new, it already exists in L4D for crawlspaces just to aid the player to move more fluidly.
I think it would make a fine addition to Goldeneye: Source and could be put to good use in places like Facility, Archives and Complex's vents and the low ceiling/crawlspace at the top level of Control. Maybe even under the ramps in Stack and Complex like in the original. I'm sure it would definitely reduce the slight frustration of manually crouching to enter vents and give even more Goldeneye-feel to the movement.