Info:
There will be 25 main mission, totaling for a 20 hour campaign, and 35 side mission for an additional 15 hours. Totaling, 35 hours of campaign based content, not including collectibles, Easter eggs or other hidden goods.
Skulls return, and are hold-able much like it was in Halo 3.
25 Main Missions all feature Chief as the playable character.
Side missions are unlocked when beating a main mission, and feature a different playable character, as they take place concurrently with the mission that unlocked them. Each main mission can unlock anywhere from 0 to 3 side missions.
All missions, main and side, will be launch-able from the main menu at anytime after unlocking them, so you don't have to worry about missing a side mission if you journey ahead.
The featured characters that will be in the side missions include:
Kelly, Linda, Fred, Jerome, Alice, Douglas, Palmer, Locke, Buck, Vale, Tanaka, Arbiter, Atriox, Let 'Volir
After beating the campaign you will unlock a new mode: Exploration Mode.
Which lets you explore the open world environments without being restricted to the campaign, as your multiplayer spartan too. the locations include, Installation 07, Installation 04C, The Arc, Earth, and Reach. As well as 1 more location I wont mention as it would be a HUGE spoiler for the main story.
Playable elites return to multiplayer with full armor customization, along with the introduction of playable Brutes, however brute customization is limited, akin to how elite customization worked in Reach.
REQ weapons and vehicles will not return in Halo Infinite, except for a select few, but they will live on as Weapon Skins instead for Weapon Skin supported guns. Most notably Oathsworn and Nornfang, which will be used by Kelly and Linda respectively.