I'd like to see custom gravity fields.

What I mean is that I assume that in most game engines down is - Z, it would just be a vector or just baked into physics calculations.

I think that 343 could make more interesting and more realistic environments if their worlds could be curved rather than planar.

If they are making large levels them maybe the curvature of the earth could be apparent. Things far away would be over the horizon, levels would have a slight curvature. This would be really apparent on small moons or big asteroids.

But even better would be levels set inside O'Neil cylinders. You could have levels with the advantages of an island level (user roaming would be naturally limited) if the level was the inside of a large cylinder. You could see all parts of the level, you could see your destination above you but you'd travel there by warthog.

You could have starscrapers, buildings that stick out from the cylinder with windows that look out: into space, the thing that the station is in orbit of, and any ships nearby. Each starscraper could have a central atrium and lobby like some hotels and other big buildings have.

At the start of a level you might have to fight your way out of one. There would be lots of levels and balconies for good vertical level design. Then you would reach "open ground" in the main part of the cylinder and have to take a hog and make your way to another starscraper. I imagine that these things have buildings that stick out inside the cylinder with impressive entrances and lobbies that lead to a big atrium in the center. Maybe you get to free-fall down the center and land in a pool at the bottom. Imagine a "glass" sided and bottomed pool that looks out into space.

There could be a train system that is suspended from the main cylinder Again, they would have windows that show the body of the cylinder above you, starscrapers, nearby ships, and whatever you are in orbit around. There are train stations sunken into the floor of the cylinder. The interior of the cylinder could be mostly park land and agricultural.

Halo was the first game that tried to visualize what it's like to be on a ring world. I think that Halo should carry on this tradition to other sf environments.

Halo 4 tried to do this with the place where the Composer was kept but because they used bog standard gravity physics and air forcefields... it just felt like you were fighting in a cave. 343 should do an asteroid environment justice with a proper curved surface and transitions from interior to exterior space.

Another fun environment would be the interior of a large ship with only emergency power and no artificial gravity. You would have mag-boots and thrusters and could start out in small rooms and corridors but make your way to large hangers where it would be suicide to expose yourself in the open so you'd need to take cover behind and below vehicles chained to the floors of the hanger. But you could also take to the walls and ceilings to sneak around enemies.