Nice ideas! Agree all of these would make the game better and more challenging!
This could be randomized or some rules set for NPCs to either ignore (not all NPCs know if an area is a trespassing zone), or what you mentioned above, or call the guards.
This was incorporated in Splinter Cell: Blacklist and it was awesome! It would be really nice to have this behaviour in Hitman to improve realism and pose better challenges.
We absolutely need this. It's currently in place at some parts of the game already. A good example that springs to mind, is the target at the cafe in the bonus mission, 'A House Built on Sand'. There are predefined "pockets" (portion of area) inside the cafe, including the toilet. Each pocket changes to a trespassing zone if the target is inside it, and changes back to "clear" if the target leaves it. This game mechanic has to be consistently and regularly present in all missions.
It's really funny when you get to poison Viktor (Paris), follow him along with his bodyguard all the way to the toilet, and then eliminate him there.