I’ll be real right now, we have two prime examples of non-Master Chief protagonists from the Bungie era, and two examples from the 343i era. That means we are comparing The Rookie and Noble Six, to Spartan Thorne and Agent Locke. And it’s extremely easy to see why Bungie’s alternate protagonists worked, and 343i’s didn’t. Bungie’s weren’t characters, they were shells for the player; which is why Master Chief worked so well in the first place. Thorne and Locke on the other hand were fleshed out. They were given personalities, opinions, loyalties, and backstories. And while the Rookie and Six were both pretty well adopted by the community, Locke and Thorne were both VERY poorly received.
That’s why.
Master Chief can’t live forever, and absolutely can’t be played by anyone who isn’t Steve Downes. The community would riot. So he’s going to have to eventually disappear from the spotlight. When he does, I hope Halo goes in the right direction, and passes the torch to another unnamed, unseen character. Who only speaks when something needs to be said. Like Chief (pre-H4), Rookie, and Six before them. And that character should absolutely be our multiplayer character. Heck it can even canonically be the same character we were playing as on Team Crimson in Spartan Ops. Since that was the one protagonist I think 343i got right, they got no dialogue at all since the story followed Thorne and Majestic instead. It was great, a self-insert for our multiplayer selves into the story. Just about the only thing that worked in Spartan Ops.
Or if they finally bring Elites back, they can turn the UNSC and Swords of Sanghelios into a proper alliance with both forces growing to occupy ships together in a combined fleet Star Trek style, and you can choose which species the protagonist is as well. Replacing references to the Spartan’s ID tag or decided military rank with the Elite’s decided rank instead. As an example, the Spartan could be “The Lieutenant” while as the Elite you are “The Majordomo”). It’d be sick so long as they can write around it so your species isn’t important to the plot. Both could even be considered a member of, example again, Crimson squad. So Crimson Three or something like that could apply to either species. Though please, if there’s a squad handle it like Reach, where they are intermittent parts of the story and aren’t constantly present or used as coop characters.
Anyway, yeah. That’s how I’d go forward once Master Chief is gone. Keep the new protagonist as a faceless self-insert for the player. Let us use our multiplayer customizations in the campaign. Keep player character dialogue to a contextually appropriate minimum. And give the story other characters to write a narrative around. Cortana, The Arbiter, the Keyes family, Johnson, the rest of Noble Team and the ODST squad were the characters the story was written about, not Master Chief, The Rookie, or Noble Six. We saw the story through their eyes, but they were flat protagonists in a larger story around them. They didn’t grow or develop, everyone else did.