what can you exactl do with a "MOD" instead of CS

[Amnesiaplayer]

I know that with custom story you can only create models maps etc, and for mods, you can create your own...things, like movement, main menu, sounds player make, but what more?!
My plans are actually beng able to have a "Snipe scope" active if you click on something by script etc.
are these things even possible?!
what I want to is having an 2d Animation, and if you stay in the waterthe animation progress further onlike if you stay 3 seconds in the water, the pic activates as a little icy look on the borders of your screen, staying in the water will progress the animation until you froze whole, if you get out the animation doesn't stop but go vice versa.
is that kind of things possible??