You do know Production I.G produced both 'The Duel' and 'Homecoming' for Legends, right? In addition to Toei animation for 'Odd One Out', I'd say they're both pretty top tier studios.
If it had some new studios then sure. Bones is great, but I'd like to see A-1 Pictures, Production I.G, Xebec, Wit, and a newcomer, NUT.AHAHAHAHAHAHA.................no. No way in hell are any of these top studios gonna waste their time on animating Halo. We already seriously lucked out with Bones on the first one. If we're very lucky again, we'll get a mid-tier studio like Gonzo or Satelight, but knowing our luck it'll probably be trash tier like Hoods Entertainment or Arms or DEEN.
Plus, if you think studios like A-1 Pictures have 'too high standards' for Halo, I urge you to watch anything after episode one of Aldnoah.Zero, or GATE, or any of The Asterisk War, tell me how good you think they are from an animation standpoint.
Sunrise are hungry enough for income they've decided to revive Code Geass of all things, so I think some M$ would talk in their direction.
That is of course, implying the project would be worth investing in at all when the games are currently under-performing.
I forgot about I.G's involvement in Legends, my bad. Toei on the other hand, is a shadow of it's former self. They've been pretty much just coasting on One Piece's success for the past decade and even that's running thin, as evidenced by them milking the hell out of DBS. I will admit though, that Seikai Suru Kado is a bright spot.
I never said A-1 et cetera had too high standards for Halo, I'm saying that they already have a crapton of shows lined up for goodness knows how many more subsequent seasons. Just see Granblue Fantasy for an example of major production delays. To devote precious resources away for making Halo would require them to sacrifice much more lucrative anime contracts, which won't make sense to them. And frankly I don't see MS throwing the amount of money required to pull some of these studios away, considering Legends wasn't much of a money-maker as well.
In short, it's good to dream, but then reality smacks you in the face with the Fist of Rukt.