The Helldivers 2 community was ready for an all-out invasion once Arrowhead announced that Illuminate would be coming in the Omens of Tyranny update. With the Helldivers retreating, Super Earth repulsed the initial attack in just one day. However, despite preparation for a massive second wave attack, none came.

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The Illuminati have settled for fighting on the outer fringes of the galaxy with relatively lighter troops that sometimes feel easier to repel than even the Terminids. That along with everything brewing in the Meridia black hole, the players sensed that something was wrong. While there are plenty of theories floating around, one I saw on Reddit is by far the scariest.
Lighting is not the real enemy
Simply put, the theory Ryukage06 presented is that Illuminate don't attack, they run away. After being absolutely destroyed in the First Galactic War, an advanced species like the Illuminate would know better than to attack Super Earth with such overwhelming force. They speculated that the Illuminate was actually fleeing an even greater threat to its existence than Super Earth.
The theory suggests that this unknown enemy took advantage of the Illuminate's weakened state after the first war, forcing them to either flee with the few forces they had left, or bait this enemy into an all-out war with Super Earth.
As for who or what this possible enemy is, they speculated that it could be “some sort of bio-horror, like the Flood from Halo or the Thing from John Carpenter's '80s sci-fi horror. [classic]. This enemy would be easy (easier) for in-game developers to create, as they only need to redesign/remake existing enemies to look like horribly mutated versions of the originals.”
Of course, this is all speculation, but Ryukage06 backed up his theories with some pointers that you can look up on his Reddit post.
Another possible new faction that players have speculated about is a race inspired by HP Lovecraft's cosmic horror. Literal ancient demonic enemies like Cthulhu and his minions would be an interesting new enemy to drop a 500kg bomb on.
Whatever this possible new enemy may be, you can be sure that he will never be able to shake our freedom or take away our freedom.

Helldivers 2
The Galaxy's last line of offense. Join the Helldivers and join the fight for freedom in a hostile galaxy in a fast, frantic and ferocious third-person shooter. Helldivers 2 features Arrowhead's best co-op gameplay to date. Collaboration will be vital: teams will synergize on loadouts, strategize for each mission, and complete objectives together.