I can certainly relate to the themes brought up, I wasn't ready to see chief return so quickly. It would have been a much stronger return even 3 years later.

That being said, based on the tone between cortana and chief at the end of h3, I got more of an impression that chief would be awoken when cortana was feeling particularly lonely or nearing rampancy.

Now I never actually expected to see that in-game just because it's not very interesting as an opening video game experience, but I was not expecting some new monumental threat per se.