Rereading the patch notes and the “Patient Zero Support” section, I’m thinking that the new mechanics, AI behavior, etc. are things that are now technically added to the entire game, but only the Patient Zero campaign actually puts them to use. The most obvious example would be shooting NPCs into bushes, but there aren’t any bushes in the S1 missions, so there’s no opportunity to use it outside Patient Zero.
They mention new HUD elements… How crazy would it be if they added a “visibility” meter, like in Splinter Cell? It’d probably work well with the lighting improvements. I’m not sure if I’d actually like that, but it’s an interesting thought. I’m guessing we’ll see at least one significant change/addition if they’re talking about not spoiling the experience. Maybe then the Season 2 stuff will be made to make use of these new things, which would also mean that PZ would, in fact, serve as a taste of what’s to come.