I mean, Master Chief would be huge as a third party, especially since he is exclusively Microsoft. But regardless of his relevancy and status in the gaming community, and how iconic it would be to have Master Chief in the game, he makes no sense when compared to Banjo and the legacy/history he has with Nintendo. I personally don't like Minecraft, I've nothing against people who do either, but something about its simplistic and blocky art style and bare bones character design just doesn't sit right with me as a character - yes, you could say we have G&W who is just a stick figure, but he isn't the amazing link between Nintendo and Microsoft in Smash - that link should be Banjo.
Like I said before, a Minecraft trophy, item, stage or assist trophy is fine by me - but no character, please