If the Arbiter isn’t featured as a significant part of the game, we riot.
Okay…give me reasons on why he should be in the game. As in, Halo universe reasons. The Arbiter was a bit busy last time we saw him.
That’s easy. The Arbiter is the leader of the Swords of Sanghelios, who are allies to the UNSC. If there’s a significant threat to fight, whether that be the Banished or something even worse, there’s good reason for the Elites and the UNSC to stand together once again. Plus, the Arbiter and the Chief are friends. Chief could totally hit him up for some support.
Edit: At the end of the day, though, there doesn’t even need to be a super logical reason for the Arbiter to be part of the plot. If they want him to factor in to the plot, he will. Sergeant Johnson tagged along with the Chief for the first three Halo games even though it wasn’t necessarily logical. At the end of Halo CE, he should have been dead, but they wanted him to be in Halo 2, so he was, and they came up with some contrived way of having him survive.