The sandbox is the most important thing in a game because it defines what a player can use in gameplay. Since Halo 2, the sandbox of Halo has grown more and more inbalanced and we come up with "useless" weapons within the sandbox. Halo, being a weapon pickup game rather than a weapon progression, should not go down this path of having weapons that are superior to other weapons.
That said, the new sandbox for Halo 4 should be modeled after CE's, but with new and innovative weapons that are balanced with each other.
The sandbox for CE was ultimately defined by this: every weapon could kill in its own way. This "way" is called a niche, and a niche is what something excels in. Weapons should NEVER
overlap in niche (be able to accomplish two or more niches to a great extent). The CE Pistol was a Mid range weapon that was only decent to use in the other two ranges, and wasn't superior in those ranges either. So, while it was capable at multiple ranges, it did not infringe on the niche of the Sniper or the Shotgun at their ranges and its niche was defined as Mid range.
Niches go beyond range, however. It also applies to effectiveness against different enemies and utility in battle.
For example, let's compare the Rocket to the Laser. The Rocket and Laser both excel in heavy damage, but have less spare ammunition to balance it out. The Rocket fires a projectile and the Laser fires an almost instantaneous high energy blast. Both do high damage against vehicles and infantry alike. The Rocket excels at being able to multi task shooting at more vehicles and infantry better than the Laser and has the benefit of a bigger ammunition supply. The Laser, though, can shoot farther, deals more damage, and can land a hit within a second at a longer range while the Rocket must wait for its projectile to make contact.
So, the Rocket can be effective against more enemies due to a blast radius and more rockets to fire, but a Laser is almost garunteed a kill and can fire at long ranges. Players, therefore, use these distinctive qualities to measure which would be more effective in a situation.
So, the sandbox of a balanced Halo game should not be of weapon overall effectiveness, but effectiveness in niche. Since a Mid range weapon is generally a utilitarian weapon that is decent at the most amount of niches (but excels more in Mid range), it thus becomes an ideal starting weapon. Making a utilitarian too effective at certain niches (such as a DMR being able to destroy vehicles and cross-map effectively) is bad for the sandbox since there is no reason to supplement the utilitarian's faults and thus it becomes the sole weapon used in the game in place of the full utilization of the sandbox like a weapon pickup game should ultimately attempt to become.
Making other weapons in the sandbox ideal to pick up for a specific reason(s) and not making any other weapon too advantageous for the user will also make the sandbox balanced and teeming with variety.