I think it would have been cool. To those who were hoping to play as your Spartan in a Halo campaign after Reach, were you expecting that when H4 was announced?
Firstly, I was never expecting a Halo 4 at all at the time. Secondly, after learning that there would be a Halo 4, I always assumed it would be a continuation of Chief’s story, thus playing as Chief.
I wasn’t necessarily expecting it, since a direct sequel would automatically mean focusing on Master Chief John-S117’s mission(s) (or at the very least, missions of other Spartan-IIs, like Blue Team, and/or specifically Fireteam Alpha-Nine - the ones from Halo ODST, or even Arbiter Thel Vadam’), but it could’ve been nice to have something akin to both Halo 3’s Co-Op Campaign characters (the random Sangheili/Elites) & Halo 4’s Spartan Ops, with our own Spartan.
It could’ve been an interesting new way to continue the Halo story, if, say… there was some sort of Campaign A.I. Co-Op mode, where the devs’ scripted A.I. would take control of Master Chief in the Campaign, for once, and our custom Spartan (along with two others?) would fight alongside Chief & take orders/directions from him (e.g. like doing pincer maneuvers, flanking the enemy, providing him cover fire, or him providing us cover fire, et c.), in order to accomplish a particular task in a mission.
(The Halo story might end up progressing in this direction, once Chief nears getting too old to fight in the field, & he has to pass the torch on to a younger HLV (Hyper Lethal Vector) Spartan, of his assessment & choosing.)
And with friends, we can have a friend take control of Master Chief, if we want to control our own Spartan.
Also, for the choice to either play as any of the Blue Team Spartans (including Chief), or our own PVP Spartan, in full 4-player Co-Op Campaign (not locking Chief to the Player 1 controller).
There might have to be an alternate campaign route in order to facilitate choosing our own Spartans (i.e. Dead Space 3) over whatever the default co-op characters would end up being, but it’s not entirely impossible - just more work.
I think, in order for us to continue being able to play as our own Spartan in future Halo games, (if the story doesn’t follow as I’ve described above) that potential Halo game will likely be another spin-off, pulled from the lore, following the story of unnamed/unsung Spartans, that faced the same/similar fates as J.D. “The Rookie” (ODST) & Noble Spartan-B312 (Reach).
[Random Sidenote:
I think, for Halo 3 MCC, 343i should replace the random Sangheili/Elite Co-Op characters with Blue Team instead (if it adheres to the lore though, of course; not sure if it does) cause I think that would’ve been the most opportune time to have introduced Blue Team to everyone. And that opportunity was just not taken. 
]
I could see it with Fireteam Osiris getting custom armors.
Then I could make the team look doofy and Buck look amazing.