we all know its coming, now heres my theory on the story.
TL;DR at bottom
THIS IS ONLY MY THEORY I HAVE THOUGHT OF THROUGH INFORMATION FROM THE GAMES, BOOKS AND OTHER PEOPLE.
Ive seen a number a people saying that the blue light at the end of the legendary cutscene of Halo 3 is the shield world Onyx.
If thats the case, then for those of use for have read the book we know that the actual planet blew up and is now a giant ball of sentinels.
This being so, how would the chief and cortana even get through the massive defense system of the shield world? in the book any nearby covenant cruisers were blown up.
If the half-destroyed frigate does get through the shield somehow that means there wouldn't be any marines and a small amount of warthogs and other UNSC equipment available during the campaign.
If this does take place on Onyx then im guessing the chief would eventually meet up with Dr. Halsey, Fred, Kelly, C.P.O Mendez, Linda, Ash, Olivia, Mark and the cryo pods with team Katana inside.(Most of these characters are spartan IIs and IIIs)
The enemy would probably just be aton of different types of sentinels. Or maybe they'll find Forerunners? Or the Flood?
Although: During the story the UNSC could send a fleet of ships because of the distress signal cortana activated. The same could be said of the Covenant. Or maybe both will come and there will be some sort of huge battle we've read about but never seen?
If the Covenant were to send some type of army and get through the shield somehow then the covenant could also be likely enemies during the campaign. But these events would be taking place after the Prophet of Truth had been killed. But it could be possible that another prophet or race of the Covenant stepped up and took control. Or who knows, maybe if we would meet forerunners they would end up being enemies and the UNSC and Covenant would join forces?
Feel free to post any ideas you have on this theory.
Again this is only my theory based off all the halo games, books, other sources ive read.
TL;DR: Halo 4 story will be about counters with the characters from ghosts of onyx and forerunners
Heres the trailer from E3 2011 Halo 4