Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
The Books:
The series is in fact a distant prequel to the Warhammer 40,000 universe
Think about it. Post apocalyptic earth. Lots of giant guns. It makes sense.- The mutant hate that lead to the uprising against the Scriven was in fact engineered the The Emperor.
- Big E also created the Green Storm as part of a plan to eliminate Traction Cities, which finally came to fruition leaving the Thunder Warriors unchallenged.
- Traction city technology would later be used as the basis for the Adeptus Mechanicus Ordinatus war machines and the Imperial Guard Leviathan command vehicles.
- Among the millions of inhabited worlds in the galaxy, there's Zayth
, whose devastated surface is roamed by "landships," each one a self-contained mobile city, that constantly hunt eachother. Since this Municipal Darwinist conflict has gone on for enough millennia that the Zaythans have lost all record of how or when it started, yet the number of landships never seems to decrease, it is presumed that someone is building new ones somewhere in the planets post-apocalyptic wasteland. The Adeptus Mechanicus would really, really like to poke around the guts of a Zaythan landship to figure out what forgotten technology makes them tick, but those landships give Imperial Navy battleships a run for their money in both size and firepower, and they have some rather forceful ways of telling the cogboys to get the hell off their lawn.
The Slow Bombs are coming
In Scrivener's Moon, the Stalkers / remembering machines in Skrevanastuut tell Fever about the Slow Bombs. They say that the weapons are due to start arriving in three to twelve years. Fever assumes the Stalkers are talking about something that happened millennia ago, during the "Downsizing." However, at least some of the Institute's technology is still operational, and keeping track of time is a pretty basic function of many devices — and would certainly be important in a place like that. And the Stalkers of Skrevanastuut seem aware of the world around them to at least some extent. They are certainly aware of Fever, and even know exactly what she is, right down to her missing spots. They do describe the Downsizing as something "recent," but the word "recent" might mean something very different for them — they are built to remember the past, after all, and they've seen and experienced nothing at all for millennia. But they may in fact still be perfectly aware that the Downsizing happened thousands of years earlier. When they say the Slow Bombs will arrive within the next twelve years... they may actually mean twelve years from that moment when Fever is talking to them.Fever's world may be in for some more interesting times.
- Confirmed. The Traction Codex (Which pertains to the Mortal Engines quartet) notes that the series takes place about 500 years after Scrivener's Moon, and that the Slow Bombs have arrived - They've completely annihilated Panama and split the Americas.
Shrike will lead the human race into a golden age
At the very end of A Darkling Plain, Shrike awakens among peaceful and welcoming humans who have allowed the Earth to grow green again. They now revere him as a repository of ancient wisdom – but he could be much more than that. He's almost immortal, has memories that stretch back thousands of years, and now finally has his human goodness restored. The humans of the epilogue already have access to antigravity, but are otherwise technologically simple – Shrike could help them rebuild advanced civilization without allowing the chaos and ruin of the Sixty Minute War to reoccur. Five thousand years later, humanity could finally be a peaceful and enlightened spacefaring power, led by its benevolent God-Emperor, Shrike.- Think of it as a nicer version of the 40K universe.
The series is a distant sequel to Global Frequency
Weaponized body horror cyborgs and carelessly abandoned doomsday devices everywhere. The Frequency must have really dropped the ball for the Sixty Minute War.