It's been a while since I've done something like this, but the posts I have made for it are either too old or simply no longer exist. As such, I will try my hand at summarizing what I consider good design for Halo's weapon sandbox, and how it balances and relates to the game at large. This is not meant to get particularly specific, although I will be using specific examples to better explain my case.
Before I get started, I feel it is necessary to define a few terms. These terms are generally well understood by this community, so I admit it being merely a formality. Nevertheless:
Utility weapon: a weapon designed to be a jack of all trades, made to be the perfect starting weapon
Power weapon: a weapon designed to be powerful, made to be desired and thus promote map movement and control the flow of a match
Sandbox weapon: a weapon designed to neither be a utility weapon or a power weapon, but somewhere in the middle, ideally a weapon that one would pick up to gain a slight advantage of some sort
With that out of the way, I will describe the desired harmony between these three categories of weapons and how weapons should fit within their molds.
First and foremost, a utility weapon must be skill based, able to engage targets at multiple ranges (and thus fill a mid ranged role), and must be the starting weapon for any standard competitive match. Power weapons must be powerful enough and scarce to be sought out by players in order to satisfy their role in promoting map movement and in order to provide a dynamic element to a match which will improve the gameplay's depth, while being limited, whether by ammunition, ease of use, or a particular circumstance, in order to allow the utility weapon a fighting chance. Sandbox weapons must also be desirable to pick up by way of supplementing the utility weapon's faults (ex. an AR is better for close range than the BR), with a delicate balance between it and the utility weapon in that neither should ultimately dominate the other by default (ex. a sword is not a sandbox weapon by virtue of practically dominating close range encounters in comparison to a BR, so it is treated as a power weapon).
This is obviously a very general set of "musts," so I will elaborate on a more specific basis with each category.
The concept of a utility weapon is there to solve the problem of off the spawn disadvantage a player will face after having died and respawned for the first time and on. Players who survive an initial encounter by default have the advantage in positioning and possibly in weaponry. The utility weapon, therefore, gives a respawned player a fighting chance by giving the player a weapon that can compete with every other weapon on the map. Such a powerful weapon is balanced out by being skill based, traditionally in the form of headshots: maintain accuracy, land all rounds, and score a headshot for the kill and you maximize the weapon's potential. The differential time to kill between headshots and body shots should be significant enough to warrant killing a target with the least amount of rounds used. The time to kill, related to the other weapons on the map, should be relatively close to that of sandbox weapons, but longer than that of power weapons.
When there are multiple utility weapons that exist in a game's sandbox, it is ideal to have subtle differences between them that make them all viable starting weapons, but perhaps more suited to different styles of play for different game modes or even different players (ex. a Magnum is better suited to smaller maps while a BR is better suited to bigger maps). This is not to say that a utility weapon should be better than another, but merely better in one respect. Effective range, magazine size, Rate of fire, and scope magnification all play a part in these differences. Whether or not the choice of starting with these weapons is static or a player made choice, a utility weapon must be a starting weapon 100% of the time in a standard competitive match.
Power weapons exist to add interesting elements to a match. They allow for comebacks, position breaking, and map flow. They also control where players are likely to remain around so that maps can be designed around this idea as well as giving players an ulterior objective within any match that concentrates the action, which serves to make the game more fun. They must be balanced in such a way that their ability to generate kills for your team is worth picking up in almost any circumstance, if not to use it against the enemy team then to merely keep out of the enemy's control. They also must be limited by respawn time, availability, among other ways (ex. the Sniper has more ammunition than most power weapons but requires headshots in order to maximize the weapon's potential, while the each Rocket is at least one potential kill but it only comes with two or four rockets at a time).
Power weapons should be easily understood and rewarding, while, at the same time, being able to be outshot and outplayed by utility weapons and sandbox weapons. They should be designed to be good, not dominating. They need to be able to drive momentum shifts while not being the tools used for maintaining that momentum by themselves. Assuming proper design, they also need to be placed on maps well enough in order to complement map design and player environmental interaction.
Sandbox weapons are not technically required for a proper Halo match. In theory, a Halo game can subsist on a good utility weapon with good map design and proper placement of power weapons. This doesn't mean that sandbox weapons should not be included. They are designed to be interesting and useful within certain niches. They increase the depth of a standard competitive match by augmenting the faults that the utility weapon may have (ex. the AR simply allows a player to put a continuous stream of rounds on a target while the BR has a time delay between each burst). Mechanics like plasma stun, automatic fire, cooldown, tracking, EMP, and other things serve to make these weapons worth picking up and should honestly be the developer's chance to show creativity. Mechanics such as those previously listed may also work (or even belong more to) power weapons as well, but given that sandbox weapons should have times to kill (within their niche) similar to that of the utility weapons, these mechanics are what serve to differentiate them.
Ultimately, it is up to the player whether or not they want to pick up these weapons. I myself and generally only comfortable with maintaining the utility weapon and grabbing power weapons as needed in more modern Halo, although I admit to picking up plasma rifles in CE because of their plasma stun, a game which demonstrates how well sandbox weapons can work.
It is important to maintain this balance in a Halo game, because ultimately a Halo game should be simple and easy to understand. The weapon and map selection are what make the game dynamic, and thus are almost directly responsible for making each match interesting. i would go as far to say that a failure in sandbox design is a bigger problem than a failure in map design, because Forge can ultimately be used to fix map design, whereas weapon design is purely in the developer. And part of sandbox balance is actually making sure the weapons feel good and are fun to use, because the feel should match what is happening. Players should feel empowered for landing a 4 shot, or getting a multikill with a single Rocket, or causing a Banshee to fall because of a Plasma Pistol EMP. Such is the reason why sandbox balance is so important, and such is, no matter who is working on Halo, something the developer needs to understand.