I like how they worked Laskey in. Some quick takes:
* Dr. Halsey and Laskey will be there to help MC in his next adventure.
* Dr. Halsey will be tantamount to reclaiming Cortana. A lot of options here. The Diadact was talking at the end and seeing as there is a biological-digital bridge--and all we saw was the Diadact fall, nothing more (remember, this guy is so powerful he can pick up MC with his mind)--so he may have her. She may be lost in some scraps. She could be with MC in that he has changed and was hearing the Diadact when Cortana could not. All kinds of oddness happening here.
* The Librarian changed MC. The Legendary ending shows different eyes which, based on the Forerunner back stories of metamorphosis, could very well be the crux of the next game. My prediction? Humans turn on MC. MC, with Laskey, has to find Cortana and get her to Halsey. Problem? MC will not only have to fight the Storm and Prometheans but also HUMANS, and worse yet, the new Spartans.
They have already set the tablet for MC disobeying directives, now that he may not be human, @#$@# will hit the fan.
* MC will be pushed to the edge and have to go it alone... MC will have have no Cortana, Laskey will get killed, humans have turned on him, the Storm and Prometheans have forged an aliance, and it will get worse: the flood will return and the Covenant-Brute alliance will rear it's head. I predict Halsey will die saving Cortana and the major twist in the story will be
the return of MIA Spartan-IIs. MC's old friends.
* We may even see a "Halo ODST" style gap filling game about the missing Spartans and how they worked together to regroup with humanity and come to MC's aid. I would also expect the Arbirture to play into this now that the Storm have aligned with the Prometheans.
I know some people don't like the story twists but I think there are plenty of options for what happened to Cortana--whatever it is will play a big role in rescuing her. They have a cast of characters, both good (Laskey, Halsey, the Arbiture, missing Spartans, the Librarian), some antagonistic (the ruffled General, the Diadact and Prometheans, the Covenant, the Storm, the flood) and some where we don't know where they will stand in a divided conflict (humans and new Spartans).
What we do know is something is wrong with MC and it probably is so bad he will need Cortana and the Librarian to get him out of this mess--and all the usual suspects (flood, Covenant, Prometheans, maybe some humans) will now stand in his way.
I like the story direction -- I like how MC is built up in the prologue. Oh, and the variety of LARGE sandbox gameplay pieces and gameplay evolution I found spot on. No complaints from me so far. I thought Reach really got away from the "here is an open play field, you choose where you go and how you beat everyone" whereas Halo 4 built on and above Halo 3. Heroic is hard and the gun variety and balance is cool--me and Castaway found we liked totally different weapons.