I like the 4 original propositions.
The swimming thing sounds cool as long as it isn't over done. I should still be able to wade slowly. I make a little mini-game out of slow-wading and just managing a horizon that lets me see clearly what lies beneath; and I like it! But for full-on swimming, I think the "pulse" motion would be cool.
New "particle" effects? The more the better. Bugs and birds and swarms of things. Excellent.
More sounds? More better too as well.
The SpecialCrouch... meh. I already do that. 'Tis left to my imagination and so I imagine it is so. To actually go to the trouble of implementing it and accomodating the cascading effects? Nah...
I don't really love the keyhole idea either. It is unrealistic (though not as much in this universe, to be fair, as it is in most of the games that have keyhole peeking already!). It is a major change too... but, then again, I do agree that since the guards are apparently going to be more aware of things they didn't usedta it becomes necessary.
More battle-skills for the guards? Sure. I still can't really work the fighting system anyway, so I need more reason to not get caught.
Fuck Stamina? Please don't add your Fuck Stamina all over the place.