Even though it'll never happen, I'd enjoy seeing Master Chief in SSB4 for several reasons:
1) MS doesn't have enough representatives to make a fighter crossover. Shooters, of course, would still be fair game.
2) Chief's used such a ridiculously high amount of tech in his career, even in-game, that he could very easily pull a Snake and not use any conventional, bullet-firing weapons. At the same time, he's still got a ton that aren't entirely explosive - Plasma Rifles (which I could see working as a a damage increaser with little knockback), the Needler, frickin laser beams, etc.
3) In addition to straight-up handheld weapons, he's also got access to several useful pieces of equipment that could work their way into a nonstandard moveset (jetpack for recovery, etc.)
4) Uniquely, despite weighing about a metric ton, he's had fairly high movement speed and extremely high jumps since the original title. Having a floaty bipedal firearm user would provide a pretty nice contrast to Snake's playstyle.
5) The one bullet-based weapon I could see making it in is the Assault Rifle, which has such future-ish sci-fi projectiles (bullets, but they've all got obnoxious yellow tracers on them) that it could earn a pass. I could even see it working as his pummel rather than being used as a distance weapon.
6) Potential for huge-range attacks (final smashes) are pretty much limitless.
I could almost see a scenario working where Chief mostly uses fisticuffs for tilts, an Energy Sword for smashes, a Jetpack for recovery and then has a couple of the more iconic weapons at his disposable that can be switched through with Down B. Neutral B could be to fire said weapon which would all have unique properties (I'm talking 4, maybe 5 weapons maximum here), and then his Side Specials could be for a juke, for example using his little Thruster Pack thing.
If you really want to go off the rails, he could have a Little Mac-styled overlay just above his % meter that has a minimal Halo HUD-looking count of ammunition he's got for the weapon on hand. So, like, you'd have limited use of your ranged / projectile weapons, and they'd all reload on a new stock.
