What would look like an actual 4th faction

A faction specific MBT with a very high drop and high damage may be interesting... Throwing powerful grenades making more damage than the other tanks, the total contrary of Vanu's Magrider long range attack/defense style.

So the Vanu's opposite tank would be very short range, resistant sported with flamethrowers and nades launcher

Catapult? No
Battering Ram? Yes

Or they just make the current factions more unique.

TR is the "standard" faction with more wordly weapons and bullets, a more generalist faction.
VS is the high-tech faction. High specialization and futuristic weapons.
NC is the prototype faction. Hard-hitting weapons combined with strange abilities and utilities and a medium specialization.

There's no low tech faction in the current list, now how a low tech faction could even the odds against the three others,
they could use forbidden weapons, or biological compounds or break treaties. Using uranium yield equivalent shells (nanitium?!), chemical weapons, napalm?
They could also scavenge abandonned weapons or spy the other factions and get their tech this way but with some drawbacks due to their lack of expertise. Their guns could jam? They would try unprofessionnal setups like 12x scopes on their faction specific standard pistol? Explosive ammo on their knife so you'd lose a bit of hp every time you melee?

That'd be cool as Vanguard and Prowler are quite long range and Magrider is top range. A battering ram would be very nerf on the paper but with the right stats it may be perfectly playable. If it's effective in its battering range, it should be able to fend off melee range infantry for example (it will be the only c4 proof tank).

Or a tank with a higher range than the magrider, an artillery?

The closest of it would be the chaos faction of warhammer, or in the design of doom's monsters, aliens with cybernetic parts and occult worships

I'd like to see the return of the Earth forces as the 4th faction.

Lore
In the year 2854, several years into the outbreak of armed hostilities on Auraxis, unidentified spaceships are spotted in orbit over the planet. Communication with these vessels reveals that they are of Earth origin, and in fact are a military expeditionary fleet much like the ones that originally deposited the current Auraxians in their corner of space. It turns out that the wormhole reopened again in the year 2836, allowing a significant force from Earth to slip through. Better equipped with the experience of the disaster of the first fleet, this group colonizes the immediate area around the wormhole and has spent the last 9 years going from planet to planet.



But instead of joy at the thought of being rescued, the Auraxian factions instead turn to the Earth Expeditionary Force with distrust and hostility. The TR were the first to make contact and assumed that the EEF would act as their much-needed reinforcements, but exchanges between the two soured quickly. At first, the TR refused to cede control of their forces to the EEF, who in turn labeled the TR as a petty military coup who had committed numerous crimes against humanity. Ultimately this led to standing EFF orders to arrest TR coup leaders, which naturally wasn't taken well by the TR.

For the VS, unfortunately it was the view of the Earth authorities that the Vanu or whoever had created the wormholes were the enemies of humankind. Among the new colonies created by the EEF, one in particular had been attacked by an unidentified force and completely razed. In the eyes of the EEF, the VS were actively collaborating with an alien and dangerous foe and would have to be destroyed.

After the original 2638 disaster, all of the 75,000 crew were presumed dead, amongst them many NC executives. In the aftermath of the incident, many investigations followed which revealed the anti-authoritarian tendencies of the NC as well as significant amounts of corruption and criminal activity. As such the NC were and still are considered to be outlaws, who after a short two-week amnesty period were to be engaged on sight.

As EEF dropods begin to pepper the surface of Auraxis, the war reaches a new phase.



Gameplay
EEF forces are significantly more advanced than the native Auraxians, but their weakness lies in cost and numbers. Being an expeditionary force that has the best of Earth technology at their disposal, their main weakness is the inability to field the cost-free clones that the TR, NC, and VS rely on.

For starters, almost all infantry classes of the EEF have a nanite cost. This reflects the greater cost of casualties that the EEF suffers from due to not having access to the rebirthing matrix. While there is always a pool of experienced, trained, and equipped soldiers from other EEF colonies to draw from, this does take more time and resources.

EEF soldiers also cannot use rebirthing tubes, infantry terminals, or vehicle terminals. All personnel and vehicles are dropped in from orbit. These must be specific areas that are clear and flat enough, known as drop zones. Additionally because of EEF orbits, reinforcements or supplies may only be dropped in waves, not individually.

EEF infantry all are equipped with some form of powered armour, making them mini-MAXes. They have access to only several classes, the Rifleman, Medic, Engineer, AV Specialist, Support, and Crewman, meaning that they lack the jetpacks and cloaking of Auraxian forces.
Rifleman- Standard infantry, armed with an assault rifle, sidearm, knife, and grenades. This is the only "free" infantry class that the EEF get.
Medic- Carbine, sidearm, knife, and medical tool.
Engineer- Carbine, sidearm, knife, and placed explosives (AT Mine, AI Mine, C4).
AV Specialist- SMG, sidearm, knife, and AV launcher.
Support- LMG, sidearm, knife, and ammunition pack.
Crewman- Sidearm, knife, and very low health (no power armour). Only EEF class that can operate vehicles.



The EEF, like most modern forces, rely much more heavily on vehicles than they do infantry. The EEF are the only faction that uses long-range bombardment as a regular tactic to destroy Auraxians from afar. EEF tanks are "traditional", but have increased resistance to smaller-calibre weapons. EEF aircraft rely on long-range lock-on missiles instead of noseguns for combat, reserving those for mopping up an enemy wing that was decimated by missile strike. In general, EEF vehicles are stronger than Auraxian counterparts, but are more expensive (not counting the Crewmen, who cost resources as well). They also tend to take more maintenance (must have constant logistical support)

The EEF have access to the following vehicles:
Light Transport- Carries 12, with reasonable armour but excellent speed. Only a single roof-mounted light weapon. The workhorse transport that can repair and rearm friendly vehicles if so outfitted.
Heavy Transport- Carries 6, with heavier armour and a heavier turret capable of engaging other tanks. Speed is a bit slower, but is able to keep up with MBTs.
MBT- Heavily armored with no gimmicks, a real tank. 20mm Basilisks do nothing. 40mm Furies do nothing. 60mm Bulldogs do nothing. C4 does nothing. Made to combat enemy tanks and ignores all else (except aircraft). Does, however, require a minimum crew of 2, a driver and a gunner.
Artillery- A thin-skinned vehicle that must deploy to fire, but can launch a number of different AI/AV munitions into a neighboring hex. It is standard EEF doctrine to bomb the enemy into submission and then mop up with armour and infantry.
SAM- Area SAM capable of targeting all aircraft within a hex and then some. Must deploy to fire. Unlike native Auraxian AA, this is actually lethal.
Ground Attack Aircraft- Not capable of hovering at altitude, these aircraft are only capable of VTOL and otherwise must maintain speed to maintain height. Armed with A2G AV lock-on missiles or free-fall bombs.
Interceptor Aircraft- Also not capable of hovering. Equipped as standard with extreme long-range A2A missiles. Relatively low ammunition for nose gun. Capable of extremely high speeds that far outpace any Auraxian aircraft.
Transport aircraft- Carries 12, though is relatively large and bulky. Simple a slightly enlarged equivalent to the Valkryie.

Dunno about the nanite cost of your EEF faction, it may disrupt the pace of the game. The history and technology in your description are quite interesting. The sams,and ground aircraft would add more variety to the game.
Why all infantry mini-max? It's so OP you wouldn't hear the end of it in the forum

Optionally, their warpgate could be pictured by their mothership, anchored at one of the corners of the map if this can help the immersion

About the EEF contempt toward rebirthing, if you had to still respawn in one, you could hear something dissonnant like how they will destroy them once the war is over!

On Shaggath's post of a NSDAP styled faction, you can spin this kind of story:
"In order to create the perfect soldier, a sectarian branch of the Vanu tempered the rebirth tubes.
To their dismay, the rebirthed looked not just strongers and smarters, but their minds were twisted.
Somes had horns, scars, pimples and other tumors. And they were incredibly violents.

Thoses mutants forced their own creators in the rebirth tubes and them bowed down to them as their new masters.
A rebirth tube can only be corrupted, and the corruption remain only if the base holding them is owned by that 'apocalypse' faction".

So the other factions would have to take all the rebirth tubes in order to drive out the 4th faction.

Civilians. Civilians for TR to oppress, for Vanu to mystify, and for NC to make false promises to. The next step would be to add cities. Civvies would run around in a panic and NC and VS would try their hardest not to shoot them and then the TR would massacre the civvies. On a more serious note, a civilian system like in ArmA with other humans, chickens, cows, goats, brids, fish, and whatnot would be cool. Of course that actually takes work and releasing a new sniper toy to shoot up your *** is higher on the priority list.

I still say Amish would make a great 4th faction.

As the story says, Humans found new planet to live - Auraxis, originally we all were TR, but new leader was a dick, so it divided into rebels NC and sectants VS. There could be original natives of planet Auraxis as 4th faction, "aliens". They would look like characters from Warframe.

Indigeneous aliens who didn't meddled with the futuristic ones... So when for some reasons the top aliens disappeared the lower ones inherited auraxis, so they would have a matching technology and not the op gears their big bros had...

Amerish for the amish? This continent will be perma locked!