The solution! Well the solution to the Elite problem. Since the multiplayer has a story just find a way to incorporate Elites in the story. Like this, there are 2 non playable captians of red and blue team who start to have a rivalry that turns into a bitter hatred of each other, and when(or If)the Infinity crashes the teams would probably split up and actually start to kill each other, and while scavenging on the deserted planet, they find a Sangaheli ship full of elites that has also crashed. But before they crashed they determined that something has pulled them to the planet. The Spartans force the Elites to choose sides or perish. Then, some (not all) spartans who see the “war” between the two teams form their own sides (green and Yellow). The “rouge” teams (green and yellow) make a secret pact to not attack each other and to work together to get off the planet. The other “main” teams find out about this and instead of trying to work together they try to work by themselves to get off the planet, alone. The Green and yellow teams start to break the pact due to disagrements on how to do things. Then some tension starts to build up,some elites even think about leaving. Then in the large final battle between all 4 teams, they find out how and why they were “pulled” here. Which was a surviving forrunner civilization summoning them for help.
Either that or they could have a spartan ops mission that is called brute hunting that lets you play as elites who are hunting brutes down.
Hope that helps! 