So the DLC is called "Home of Light", and involves around Terracorp and their allies - Terrans. Focusing on mining and technology. 
The system is under threat by Khaak. Yup, they were almost wiped out, but not completly. 
There's a pirate faction, called the "Atlas", they're enemies to the Terrans and Terracorp. 

There may be a new plotline, not sure what it's about though. 
I've read about some jumpdrive anomalies, but I can't tell if they're just plot events, or can occur normally. But it seems similar to the Unfocused jumpdrive from X3. 

Tons of new stations are incoming. Terracorp officies, civilian habitation, terran colonies, pirate stations, civilian blocks along highways. Science labs. A working Aqueduct water collector (there's one in Maelstorm already, but the xenon control that area). 
But also, a warehouse station. It seems like the player can build one too. 

New weapons are coming too. Plasma Cannon Mk3 and Railgun Mk1. Not sure what they do. 

A new transporter ship, M-sized, will be added to the Canterran shipyard. Not sure if it's related to the DLC. 

There are traces of Boron and Paranid in the files. But I think that they'll only come as NPCs on stations and crew, they won't be a faction and won't have their own ships/stations/space.
- Earth Ultimatum IV.