Campaign is planned to be 3-5 times longer than Halo 4. No more linear , going open world.
Master Chief has gone rogue, has own ship and can visit different planets (6-8 planned). Each planet has several regions with main objectives and several side missions. ( Think of Wings of Liberty or Dawn of War 2 cmapaign)
Core gameplay is still Halo (2 weapons, grenades, melee; "30 seconds of fun"; sevaral different enemies) but there are RPG elements added to it: character progression (mostly items like forerunner relicts you will find that make Master Chief stronger or will give him more ablities), not really loot, but you will have to acquire weapons first, before you can use them in more missions. Weapons are stored in the armory on the ship and chief can choose his loadout, which he will use on the mission start. He can still pickup and use the weapons he finds in the different missions.Loadout weapons are basically the same you have in Halo 4 Multiplayer (you cant start with a rocket launcher, but you can call in those "super weapons" on a mission like in Infinity Multiplayer; difference is this is a limited ability, at "low level" you can call one time, this can be upgraded over time)
There will be more weapons in campaign than in Halo 4(and yes more "alien" weapons; not just human weapons with a different coat)

What do you guys think about this.