1. The Didact must somehow come to realize his mistakes and align with humanity. Remember, in the books, he isn't exactly the opposition; the Builders are. He will become a more meaningful character, because the Covenant (including the Arbiter) revere him, and the humans hate him. Chief will hate him initially but once Cortana is revealed to be still alive, she tells Chief it was her own doing, not the Didact's.
2. Spartan Ops and Firefight both make a return. Spartan Ops will follow the plot of the Storm Covenant and Arbiter's war against them, with the Infinity assisting yet again. Firefight will take place from Fireteam Crimson's POV, and will have a strong connection to the Spartan Ops story.
3. Spartan Ops missions will be taking place from Fireteam Crimson's POV, which will involve assisting Sangheili forces against the remainder of the Storm Covenant. There will be a wider variety of objectives, such as 'drive here and collect resources' or 'fly around and deploy scanners in an unmapped area'. There will be missions that will have Crimson take command of Mammoths or Pelicans to deepen the variety of gameplay.
4. Firefight will have deploy-able defenses, such as turrets, vehicle pads, and cover barricades, similar to Dominion. Resupply drops will take place every 2.5 minutes, and games can last up to 1 hour. There will be an AI, likely Roland, that will tell you when reinforcements are incoming and what kinds of enemies you can expect. At the end of every set, a boss (such as a squad of Wraiths, a Scarab, or two Phantoms) will appear and you will have to defeat it to advance. There will be larger maps and a smarter AI system than Firefight 2.0.
5. Armor & Loadout customization will be much more meaningful, rewarding and in-depth, with hundreds of armor pieces that give you small stat bonuses. Loadouts have many more options including the existing Halo 4 selection. Weapon mods will boost weapon performance and give players a more varied experience. There will be over 30 different armor colors, customizable into four sections - primary, secondary, tertiary, and detail. Armor skins are universal and can be applied to any armor set. This extreme degree of customization will ensure that no two Spartans play the same way.
Well, my brain is spent for tonight. I will add more to this post tomorrow, including my wishes for specializations, vehicles, weapons, multiplayer gamemodes, and improvements.