I want to see new main characters in the future Halo games (adds more variety to the series and expands the universe), but I feel right now, since they stuck with Chief all this time, they should at least finish his story first. Let him finish his story in Halo 6, and then introduce a new cast of Spartans to take the front lines, with Chief's epic legacy getting mentioned from time to time. He can either sacrifice himself, retire, or die of old age. I wouldn't mind a fake "death" where he leads almost everyone to believe he is dead, so he can finally leave his military career and go onto a normal life (if he just said he retired, no one would accept that crap and just keep bothering him to keep saving humanity).
I imagine Locke as this overly aggressive soldier and cocky trashtalker, but way more silent and subtle compared to Sergeant Johnson. I guess it'd be kind of comparable to Dante from the Devil May Cry series, where he trash talks a lot, but he says in this chill and laid-back manner. Most of the UNSC would turn out to really dislike Locke's aggression and trashtalk, but they still give him all the important assignments because he's one of the most capable soldiers there.