This is just what I think a cool Halo 5 story or plot would be. Obviously I haven’t thought up all levels or details, but just hear me out.
Master Chief (Single) with Blue Team (Co-Op)
Agent Locke(Single) with Locke’s Team (Co-Op)
Arbiter (Single) with Arbiter’s Elites (Co-Op)
Jul 'Mdama (Single) and his Elites (Co-Op)
Chief is getting strange messages and recordings from Cortana coming from ruined Ark. At the same time, Jul 'Mdama is attacking human planets and trying to restart the/his Covenant. Jul is also waging war with the Arbitar’s Covenant creating tensions between all Elites and even the UNSC. Master Chief and Blue Team are tasked with hunting down Jul, and killing him. Chief is given a new AI named (whatever, I like Quorra, so that’s what I’ll call her.) Immediate;y Chief dislikes Quorra, even though she IS helping him, and she understands his connection to Cortana. Chief continues to use her, much to his displeasure. Chief asks the Arbiter to be in charge of finding Jul, while Blue Team hunts for Cortana. There are a few levels on the Ark, maybe another Halo, maybe in space.
When ONI/UNSC finds out what Chief is doing, they task Locke and his team to find Chief. Locke investigates Lasky, who leads to the Arbiter, who agrees to help Locke, as long as Locke helps him wage war with Jul. (Leading to the Halo 2A Cutscenes.) Although Arbiter wants to help Chief, ending the slaughter of his troops, and stoping Jul is more important to him, so he decides to use Locke as a weapon. Locke realizes something is wrong, but plays on it, to gain Arbiters trust. Couple of levels waging was against Jul’s troops.
Meanwhile, Jul is attacking human planets that are being given as targets by Dr. Halsey, when his troops find a single that directs them to a possible weapon on the Ark. After a few levels of playing as Jul waging war on a planet with humans and Locke and Arbiter, he retreats and they go to the Ark.
When arriving at the Ark Chief goes out on his own, and finds that huge ship you see in that 2013 E3 trailer. It raises a buried Forerunner City, and Promeathians spawn and attack Chief. Blue Team comes to his aid, and they fight until Jul 'Mdama arrives. The big ship thing is the weapon Jul sought, but also the thing sending Cortana’s messages. Locke and Arbiter show up and everyone is tense. Arbiter pulls a pistol on Chief stating that he is his target. Chief aims one back at Locke for defense. Chief and Jul also pull guns on each other. (This is all dual wield plasma pistols and pistols at each other.) Jul because he want’s the “weapon”, and Chief because he wan’t Cortana. Jul also aims his second gun at Arbiter his mortal enemy, who returns the favor. Arbiter aims his second gun at Locke, showing his loyalty to chief, Locke saw this coming, so he as well pulls his other pistol. Mean while Blue Team, Locke’s Team, Arbiter’s Elites, and Jul’s Elites are all aiming there guns at each other. Basically a giant Mexican Standoff. Then everyone realizes that the odd one out is Jul, so they all aim their guns at Jul because he’s the common enemy. But before they kill him and his troops…
The ship opens up, and the deformed and burned Didact appears. (After Halo escalation he pretty much died, but let’s say this machine was fixing him or something.) He reveals that all those present are his greatest treats, and now he has them all in one place. He releases thousands of Promeathians and even some surviving flood spores! Jul’s army, Arbitars Army, and the UNSC all show up at the worst time. (Basically some troops are consumed by the flood.)
Locke is more interested in his team’s safety so he let’s Chief go. Chief orders Blue Team to leave as well. Locke, his team, and Blue Team fight through the Flood, Promethians and Jul’s army to get off the Ark.
Arbiter chases down Jul, fighting through his Army, the Flood and Promeathians. They have an epic fight which results in Jul actually winning, and Arbiter put to shame. Jul escapes, and Arbiter is now doubted by his men. Arbiter leaves the Ark and promises to find Jul and kill him.
Chief fights through all the remaining enemies, and encounters the Didact. The Didact hints that Cortana was alive, but Chief realizes he is lying. After a huge boss fight, and waves of enemies Chief is defeated by the Didact. Quorra asks for Locke’s help, and before being killed, Chief is picked up by Locke’s ship. On the ship, Locke agrees to help Chief kill the Didact, but in the end will bring Chief in. Chief agrees. The game ends with Arbiter being a dishonored leader, Jul being a war hero for his Covenant, and Chief and Locke teaming up for the greater good.Chief also thanks Quorra for saving his life.
After Credits the Didact is show inside his new ship creating and organizing an army of Promeathians. The flood are running lose on the Ark. The Didact states that “YOUR plan worked, almost. But I still want that human dead.” A red, corrupted Cortana phases before him laughing. She has been recreated to hate Chief and the humans, but retain all the knowledge of humans. She says “I want him dead too.”
So what do you guys think?