So, I came up with the idea if we can somehow implement other mechanics.

Jumping and Crawling. For example: The player jumps on the box/crate and reaches the vent by crawling to find any other way leading through somewhere (and also if the player crawls through vent, he could hear two scientists or guards talking in any spot).

Leaning. Use them when you hear an MTF or SCP. Lean, see what is it doing.

Sliding. You run, and quickly press Ctrl, then slide. A good use for hiding from hostiles, while you slide from spot to spot.

Thoughts, suggestions?

I like the idea of crawling. It would give more of an edge than just crouching. Not sure about jumping though, if it was done right and realistically, then definitely.
I feel the leaning would be perfect, especially when you encounter MTF's and SCP's

All of my yus.

Go all freeman style in the air vents.

I like some of these ideas, Q and E could be binded for some Amnesia-style leaning mechanics, which would be nice for avoiding the MTF.

Not sure about jumping and sliding, they wouldn't be too useful. Crawling could be achieved by holding ctrl for a second, and you could fit into areas like under desks in the offices.

Q and E could be binded for some Amnesia-style leaning mechanics

Ironic I use q to run and e for inventory
However they are useful keys for other mechanics.
I like the idea.

Sliding sounds complicated. Dolphin Diving would make you look like a badass, but it'd hurt you unlike in Call of Duty.

I like these ideas, they sound like they would be pretty good in this game.

I only agree with crawling. Jumping takes all the horror when the player starts to jump everywhere

Maybe not jumping, but something more realistic as climbing? It´s weir that a Class-D doesn´t have the abbility to climb. But there aren´t really much places to climb/jump
Also walking (CS walking is a good example), it could be usefull to sneak up the MTF.