A well written story from Walters and Halo 4s writer isn't going to happen, come to terms with it before you get too hyped.
I've said it before and ill say it again - Halo 4's story was actually very well written. I'm beginning to think that those that think otherwise probably just dont understand the story, because if they did, they would see why it was well written. Halo started as a mindless FPS, but over a decade it grew to have an extended lore that is deeper and more extensive than Mass Effects. Now, there were two types of Halo fans: CoD shooter fanboys and those who are like us, and love the universe and lore of Halo. And they had to appeal to both, at the same time, or Halo 4 would not be a commercial success.
The story of Halo had been heading in the direction of the Reclaimer saga for awhile, and the background of the story was laid out in subtle clues in the prior games, and fleshed out in detail in the novels - especially the Forerunner novels. And now, in order for the story to progress and humanity to become the Reclaimers, they had to introduce characters from the Halo lore which, again, is way more extensive than Mass Effect's.
That's a daunting task. I think the writer of Halo 4 succeeded, overall, although the story stumbled at times - such as during the Librarian's reveal - which no doubt had every casual Halo fan going "wtf is happening?"
And overall, he succeeded in making Halo 4 what it was supposed to be - a transition of a game. The transition between the action heavy, story light prior Halo games with a massive out-game lore, and the action and story heavy Halo games of the future, starting with Halo 5: Guardians. Halo 4 was a bridge between the two. And I think it was a narrative success, overall. Any Halo fan knew that Cortana had to die and the Ur-Didact had to be introduced to move the overall Reclaimer story arc forward. And that's exactly what they did. He didn't shy away from controversy, and he masterfully wove Halo 4's story with pre-existing lore, remaining truer to the lore than Bioware ever did with Mass Effect, and appealing to both casual gamers and hardcore Halo fans at the same time.
That's the kind of guy that knows how to work a new story into prior rich sci-fi lore. He was the perfect choice for Andromeda, honestly, and I think in retrospect a few years from now, most people will agree.