The thing I'm by far the most disappointed by is New Cortana. She is so incredibly lame.
So you make your entire trilogy (or whatever it ends up being) about Cortana, her fall and her relationship to master chief and how important they are for each other. And then, after she was ripped away from him and turns into a villain...you just give Master Chief a new AI that looks and sounds almost exactly like Cortana?? And you make her the most anime trope ever, the endearing, naive cute girl everyone loves? "Maybe it's a friend!". Come on.
The novels are full of interesting, fascinating AIs. Did you know that they can take whatever form they chose? Probably not, because in the game they either have to be young, hot women or, if they get really fancy, men. This could've been their change to give MC, if they have to give him one, a really different AI! One that challenges him and presents a completely new dynamic compared to what was done before.
Instead, they just make Cortana but cuter. I'm kind of dreading my expectation that she will somehow fuse with Old Cortana at the end or something along those lines, so that they can just give you Cortana back and basically reset and ignore the entire dynamic and story built so far. But even if they don't do that: It just undermines this entire storyline. Why even go through any of this if you're only idea is "Uh...just..give him another Cortana I guess?"
They make a point of presenting these AIs as infinitely complex constructs with as many different forms. And what do we get in the games?