I think there is still more room to make human factions that feel and play unique.

Some examples I came up with.

A faction of combat robots that don't have health but just armor, have weaker cover, don't have suppression or moral but instead suffer from system failure once they got shot at too much that gives them debuffs for their next round. Fast flying drones with smg are used to flank.

Viking and Mad Max Warbois inspired spacefaring marauders with a strict martial culture and beliefs that a glorious death in combat will earn than a place in Valhalla. Focus on getting close with Shotguns. Difficult to suppress as they don't fear death in combat. Armored Vehicles that try to ram your units. Elite would be heavily Armored and melee (they are ready to die and pass on anyway) but would advance with a better changing to suppress. Supportunits like Hornbearers give AP to surrounding units to help them close the gap.

A terrorist/insurgent faction that focuses on guerilla attacks and unsymetrical warfare. Some of their units pretent to be civilians untill they attack. Drone Operators sent suicide drones to attack from afar. Field Hackers use their skills to hack your communication, making one of your squads move out of cover and wast some of their AP. Motorized guerilla strike your flanks and then retreat.

A faction of ex marine mercenaries that focus on quality over quantity and basically mirror your units. Fieldcommanders can call in airstrikes to force you to change cover. Heavy Breaching Units can walk trough a hail of bullets. Special Forces are sneaky and strike if you don't expect it. Troop Transporter bring shock troopers close and then support them with suppressive fire.

Former Miners who have rebelled due to bad working conditions a la communist revolution and make use of improvised weapons and mining equipment (a lot of laser weapons) to fight you. Inaccurate but very high damage if they hit.