Campaign & Story — what you're fighting through
The sequel's campaign: 15 missions, four biomes, three enemy factions and a returning bioweapon. Confirmed facts from Wikipedia, GamingBolt and GamingBolt — with rumors clearly labeled.
The setting
You are a Colonial Marine deployed to the Storr Boon's Mining Facility on planet LV-558, fighting Xenomorphs and hostile synthetics across 15 missions in four biomes (Wikipedia). Between missions you return to the USS Endeavor, the hub ship with vendors and squad services.
The Endeavor crew
GamingBolt's hands-on names your command structure aboard the Endeavor: Captain Ko, plus crew members Raptor and Shipp. Your player character — callsign Hotshot — is voiced this time (GamingBolt; GamingBolt), so the fireteam banters back.
The Pathogen returns
The black-goo Pathogen — source of the first game's most disturbing enemies — is back as one of three enemy factions, confirmed in a Comic-Con interview with Cold Iron's Jake Kriegman (GamingBolt). Expect Pathogen mutants like the Harbinger alongside classic Xenos and Weyland-Yutani synthetics. AvP Central's tracker also ties a Queen appearance to an Engineer location in the launch-day trailer.
What's rumor vs confirmed
Structure at a glance
15 missions across four biomes (Wikipedia), a hub-and-mission loop aboard the Endeavor, and dedicated Horde maps separate from the campaign (DSOGaming). For the opening hour, see the first mission walkthrough; for what's documented per-biome, see missions & biomes.
Sources: Wikipedia, GamingBolt — 10 New Details, GamingBolt — 15 Things, AvP Central, DSOGaming.