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 Defense mission is returning to the series.
XCOM diehards lamented Enemy Unknown's lack of Base Defense—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.