Rather than introduce new species who come out of nowhere with no existing history to bounce off from, the writers are better off expanding upon existing species and factions in the setting and play with the dynamics these species can play off with other alien races or factions. The Yanme'e for example could really use some development. They are a species that's been a part of the Covenant for thousands of years and would almost certainly be deeply integrated with many different parts of Covenant society. Malurok for example is a world colonised by both Sangheili and Yanme'e who hold joint control over the colony. How do these races interact, and how did the different cultural beliefs and traditions affect how the Sangheili and Yanme'e developed as a people?
I actually once read a cool theory (I think it was from @Quirel IIRC) where the armour used by Sesa 'Refumee's Heretic faction was using armour inspired by Yanme'e anatomy. Looking at the armour design compared to the Yanme'e, I can see how that theory holds up, and I like the idea that the armour used by the Heretics in Halo 2 was designed on Malurok by Sangheili who have close ties with the Yanme'e.
All in all: focus on exploring the existing species in the Halo universe first.
Corporal Megan Carter, Third Battalion Fifteenth Marines, startled when PFC Matt Barthol cut loose with his MA5C right behind her. She had neither fired a shot in anger nor seen the Covenant up close before today, but after that last assault, she'd be happy to never hear the crack of a gunshot ever again. But she was still here, on Reach, and somehow she knew that the fighting wasn't going to end until the planet had fallen.
PFC Barthol flicked his selector switch to single fire and kept plinking away at whatever he saw in the haze, and some of the others in the squad joined in. Carter hit the dirt and scanned the wrecked city, but she couldn't see anything. Neither could the Staff Sergeant. He shouted for identification, and when nobody replied he called cease fire.
"Barthol, what was that?"
"Contact, sir. Drone."
"A Drone," Private Hjande Gonzalez muttered in disgust. "I just saw something moving out there and Barthol started shooting, so I-"
"You obviously weren't thinking," Staff Sergeant Kosiak snarled. "Gonzalez, next time you don't fire until you get a positive ID. That could have been one of the other squads out there. Barthol, next time you point out a target before you open fire, just so the rest of us know you haven't lost your nut. Am I clear?"
Both Marines answered in the affirmative. Carter said nothing. Neither did Corporal Ernesto Vidal. He kept walking with a glassy-eyed stare. Carter wasn't even sure he noticed the gunfire. The rest of the depleted squad had to catch up to him to continue their patrol ahead of the bulldozers, out where bomber runs had turned the forest around Orbital Defense Generator Facility 279 into a cratered moonscape. Not since they'd left the facility's perimeter had they seen any surviving humans. Now they were looking for surviving Covenant. When they found any, Vidal put them down so they couldn't report back to squidhead command or suicide-rush the dozers. Hence the glassy-eye stare. He used a plasma pistol for the job. Bullets were scarce enough.
"Just a Drone," Gonzalez muttered for the fifth time.
"Drones can be deadly," Barthol shot back.
"You probably scared it off with the first shot," the Staff Sergeant said. "Drones are worker caste. Not much smarter than a dog. Grunts at least get used for cannon fodder, and you've seen how stupid they are. If Drones made useful soldiers, the Covenant would use them."
"You ever fight them before?" Carter asked.
"Yeah, on Sargasso. They had plasma weapons, but they didn't know what to do with them. Just don't let them grab you, and you'll be fine."
"Claws like that, a Drone could tear you limb from limb in a heartbeat," Vidal spoke for the first time in an hour, startling them all. "Just like a Brute. Or a Jackal. Or an Elite. Or a Grunt. They're all freaks with too many claws and teeth. Why don't the Covenant assimilate the normal aliens?"
"Can it and keep your eyes up," the Staff Sergeant said. His eyes were firmly glued on the haze to the east.
Carter looked back to the ODG, a massive slab-sided building surrounded by annexes and armored fusion reactor sarcophaguses. There had been more to it this morning. To the north had been the massive dishes that transmitted the energy up to the ODPs until the last round of upgrades. To the west and the south was the small city of warehouses and depots and housing that had grown up around the facility over the years. Gone. Leveled. Ceded to the Covenant and then burned and blasted from the air. Damn fools had been so fixated on taking out the generator that they hadn't seen the bombers coming.
She gave a silent prayer that the Covenant would make that mistake twice, hefted her rifle, and kept walking through the haze that turned Epsilon Eridani dull and red.
"Smite my eyes, should I ever be so blind," Surakhol 'Refumee swore. Humans thought like things that crawled in the dirt. They walked along their streets even after their buildings were reduced to rubble, they hid behind cover rather than beneath it, and they never looked up. No, they looked up, but only with cursory glances. Not with the same intensity as when they surveyed the horizon.
If only they would, then Surakhol and his cohort would have a proper fight on their hands. As it was, this would be too easy.
The flightmaster called out the target, a courtyard nestled between two buildings that a prisoner had identified as reactor sarcophaguses and a long building with a semicircular roof. Surakhol acknowledged the order and dove down through the haze. He alighted on a scaffold around the southern sarcophagus, right in front of a Human who wore a yellow helmet and a stained blue uniform. The human goggled and opened her mouth, but Rikekikict, Surakhol's right-hand wingman, tackled the woman to the deck and drove its claws through her throat before she could scream.
More of the Malurok cohort arrived, on this building's scaffold and the one across the courtyard. In the moment before the slaughter, Surakhol surveyed the courtyard below. Humans bustled back and forth, carrying boxes of their metallic cartridges and bodies on stretchers. Two pits had been sunk into the ground, each one with an artillery piece and its attendant crew.
Surakhol seized the dying woman by her cloth uniform and threw her into the air. She spun end over end and crashed into the cab of a truck.
All activity in the courtyard ceased. Scores of pale faces looked up for the first time and truly saw the danger that lurked above.
Some of the 'Refumee ranks carried carbines, and they cut the fleeing Humans down with quick accurate fire. That wasn't Surakhol's way. He was a Malurok warrior of the old order, trained by the Yanme'e sages to fly and fight on the wind. He drew a pair of plasma rifles and stepped out into open air.
He landed on a gantry and ran down its length, spraying cleansing fire into the Humans who passed below. Some of them stopped short and raised their weapons. Bullets rattled off the warrior's shields, and he returned fire. More bullets cut into his shields from all around him, so Surakhol leaped again, feathered his landing, and alighted on a stack of crates. Kuwik'tsh, his left-hand wingman, was already circling around, peppering those Human soldiers with his plasma pistol. Rikekikict rebounded off an artillery piece, leaving two flares burning blue in its wake, and he too joined the fight.
This was the whirlwind, the true fighting method of Malurok. Sangheili and Yanme'e brethren working as one, always flanking and outmaneuvering those that would stand against them. Few could stand against it. Few who were caught in the middle of it knew which way to turn. To fire upon one of the brethren was to turn one's back to another.
In short order, their work was done. The signal was given. Though Humans still lived in the courtyard, Sangheili and Yanme'e spread their wings and flew on to the next. The slaughter continued in one courtyard after another, and the Malurok cohort always relocated before reinforcements could arrive. The Human garrison was thrown into pandemonium, unable to deal with the raiders or the wounded.
And then the cohort vanished.
Surakhol and his brethren were safely far away when the demolition charges planted in each of the scaffolds detonated, breaching each sarcophagus and destroying the reactors inside.