The Separatist Grunt
The Unggoy have had a hard history with the Covenant, resulting in a near-glassing of their own home planet. -But it were these events that made everyone realize just how strong they can be. Unggoy were allowed to use weapons on the battlefield, and were turned into infantry. Later, during the events that took place on the halos, a group of Unggoy joined the Heretics when they found out the truth about the halos and their purpose. Shortly after, the Elites and Humans formed an alliance for one last fight in the Human-Covenant war. The Elites were among the separatists, in which the remaining Heretic Unggoy were in alliance with, as well.
In the years to come, a bunch of catastrophic events will start happening. Requiem is involved.
Over the years, the Unggoy (Grunts) have been trained differently on the side with the Separatists and the Humans. The grunts have been trained to be braver and smarter on the battlefield. They are not slaved and they are not treated as cannon fodder. On this side there is no Deacon, as well. Instead, grunts are now taught spirituality in people as a person, rather than spirituality in the ways of the covenant. (Any grunt can be a Deacon)
Grunts are usually not built right for combat, so they are given the choice to fight. Those on the teams can usually be spotted doing work and helping out on corvettes and UNSC ships. Those on the battlefield are more built for the situation. They are more focused when they need to be, and going Kamikaze (suicidal) is not the default give up option. Suicides only happen in noble situations, where otherwise it’s a useless decision. These type of grunts are also known to be more brave.
Armor
These grunts where black-coloured armor, with a secondary colour to go with it. Their armor is more comfortable than their Covenant counterparts, and they can wear armor on the wrists and elbows. They also have an upgraded version of the Heretics’ methane re-breather apparatus on the grunts’ backs. Like the Covenant’s upgraded nose tube for breathing these grunts have one, as well. It is smaller and more closer/attached to their exoskeleton, and runs along their right cheek, leading to the methane re-breather behind them. The Covenant grunts carry their nose tube along their left cheeks.
The methane re-breather device is more straight going down the backs of the grunts. This allows them a lot more freedom to use their arms and legs in their natural ways, and even climb trees and objects as if they had no apparatus. Because of its non-clunky shape, it also weighs much less and allows the grunts more flexibility in general to move about in their surroundings.
Grunts working for the Separatists/UNSC tend to wear armor-matching hats/helmets that look to be made for humans. They’d wear helmets, military caps, and other hats (To appeal to the gaming audience). Sometimes they choose not to where one at all.
Forgive my animation skills
Overlay animation on a Heretic Grunt
Overlay animation on a Halo 4 Grunt
Originals
Heretic Grunts
Halo 4 Grunt
Note: Because of the armored casing covering the internal methane re-breather, and the way it has been designed, their apparatus is much less likely to pop off.
Weaponry
Separatist Grunts are trained for any weapon - excluding energy swords. They can even be known to use UNSC weapons on the battlefield.
Common types of Separatist Grunts:
Infantry Battle;
Weapons: Needler/Carbine/Assault Rifle/Magnum
Infantry Heavy;
Weapons: Fuel Rod Gun/Rocket Launcher/Magnum
Scout; (Shown grunt above, his armor -hat in particular- in my view represents a scout)
Weapons: Concussion Rifle/Sniper Rifle/Magnum
Grunts can ride/pilot vehicles and handle turrets.
Note: 343 Industries may like to give the Separatists colour changes to their Covenant weapons… But that’s only based on their decision.
Separatist Grunts are much more focused than the average Covenant Loyalist Grunt. This gives them the opportunity to become scouts and snipers, as being in a calm place can help. That is why their armor is black, is because they are a much more tactical grunt than just being thrown out on the battlefield. These type of grunts are great for missions at night, seeing that they can blend in and move around trees and maneuver between many things.
They are a social species, so they bring happiness and fun and stuff to their allies. I guess you could say they’d go party animal if let loose for too long.
Grunt civilizations are upgrading. It’s time for some payback on those evil Kig-yar (Jackals).
How’s the idea? I hope you get a good view on trying to imagine this kind of grunt. And yes, I guess you could say that they’re a bit of an ODST-styled grunt. 
So would you think that the Separatist Grunt should look like this? And not have that huge Covenant-styled methane re-breather? Should they enter as allies in future Halo games?