You cant' just 'slow down' animations and movement globally though, you would have to change every single script for every entity in the game, in addition to engine changes to somehow 'slow down' physics (which isn't really feasible in itself because there isn't a 'slow down' setting in Bullet anyhow, the closest you could get would be to set the default gravity to a very low value and the default damping to extremely high and even then that probably wouldn't be very realistic or predictable).
One thing that would definitely ruin the effect is that the AI would still fire their weapons at the same speed and as there aren't really any 'bullets' in the default weapons system so you still wouldn't be able to see them or dodge them.
So to recap: Engine changes to 'slow down' physics, a huge amount of scripting to allow movement and animation speed to be 'slowed down' and somehow replace the entire weapons system with one using physical 'bullets' and 'real' melee weapons.
I think Lee would ROTFLHAO!
Oh and of course I haven't even considered particle effects, lighting and sound!
Been there, done that, got all the T-Shirts!