I’m sure it’s way too late now for Infinite but maybe a possibility for a future Halo game. Remember on Resident evil 6 how people playing two different campaigns would be matched together to fight a boss? I thought that would be pretty cool for Halo. Two sets of people playing a separate campaign, not on the same server all the time but it links them through matchmaking or a friends list and sends data to each other’s server affecting their current playthrough.
For example if the squad of Spartans destroy a banished communications tower. The ODST squad will be able to assault a banished recon team without them alerting the nearby base resulting in them not dispatching banshees to the location. If the banshees are still at the base then the ODST have the option to steal them.
An alternate mission is now available. Now they can fly up to a banished ship that was about to descend upon the Spartans, destroy it and use it’s drop pods to fall down to the Spartans who are under siege by the flood or a different faction, They’ve now joined their game in a very heroic/dramatic fashion and then it turns into a huge 8 player firefight match. If they can’t fly up to the ship then they must drive over (loading screen) and have to deal with the ships reinforcements as well as the other faction. Just little things to change the story up a bit and make each playthrough a little different. Plus it would be like two games in one.