The 30 new achievements for XCOM: Enemy Within — the expansion for last year's XCOM: Enemy Unknown — slipped out to Steam a couple of weeks ago, but only now have I noticed what they describe. From the list, we can assume that a Base Defence mission is returning to the series.
XCOM diehards lamented Enemy Unknown's lack of Base Defence — a mission type in which aliens land near XCOM HQ and invade it in an all-or-nothing fight where failure or abandonment means the end of the game. The achievement "All Hands on Deck," earned with "at least 4 kills with XCOM Base Security personnel," points to its return.
XCOM: Enemy Within will feature genetically modified soliders, mecha-troopers, new maps and a ton of new baddies, and when Firaxis unveiled it last month developers there said plenty more was due to be announced. I reached out to folks at the studio and 2K Games for any kind of word on this but, to be honest, I really don't expect a response. They'll confirm it with full fanfare sometime in the next month.
A human enemy faction also sounds like it's going to be a part of Enemy Within; achievements for killing two enemy medics, or snipers and two mentioning a "new global threat," would seem to point to this.
XCOM: Enemy Within will release for PC, PS3 and Xbox 360 on November 12.
XCOM: Enemy Within Achievements [Steam]