Tbh I really loved your custom SPARTAN in Reach's Campaign.
Certainly added to the replay value, that's for sure. Can't tell you how many times I loaded up the intro just to see my shiny new helmet.
I really didn't mind the split protagonist perspective in Halo 2, it helped flesh out the story, and gave firm context as to why the Covenant was splintering. But a whole new guy? No, I don't think that'd work out oo well. Not with 343 wanting to give John more Characterization in 4. A new protag would kinda undo what 343 has done so far.
Finding it, though... that's not the hard part. It's letting go.