I really do hope Cortana comes back. I think it'd be good if she wasn't in 5 and not back until the end of 6. You can then start to feel that gap that MC would be feeling, that emptiness of knowing she isn't there to guide you or anything. And then, after he has saved the galaxy from total destruction, the Precursors return, who have been watching over him all this time, and for his "reward" Cortana returns as a human, and they live happily ever after