Chief as the only playable character just makes for that Halo CE/Halo 3 feel they're seemingly chasing (to some extent).
I would love to see Arbiter take an important narrative position as a supporting character, but I don't think we need playable Arby missions. I'm not necessarily opposed to them, but I think I'd rather see a Halo 3 situation where Co-op players get to take on other fan favorite roles instead of seeing double on MC. This could be even cooler now as players 3-4 (I don't think they've confirmed 4-player co-op locally or otherwise in the campaign yet, so I'm spitballing here) could play as characters like Palmer and Buck (Can't decide honestly if Palmer or Locke would be more divisive, but Buck's a great safe bet) instead of random elites.
Maybe the co-op players could have a pool of characters to choose from in the launch menu kind of like Gears 5 does? Only including members of Blue Team, Osiris, and some miscellaneous characters from the Halo Universe (Unlocking playable Noble Team for co-op by beating the campaign on Legendary would be a spectacular prize).
That's all highly speculative and doesn't have any evidenciary support, so it's just meant as a cool suggestion for adding a lot of replay value to the campaign and for making it all feel cohesive with the history of the series for longtime fans.
I think Infinite could learn a lot else from how the Coalition has structured Gears 5, too. I know the MS owned studios keep a fairly consistent line of communication, so that's very encouraging!