XCOM: Enemy Within DLC (EU2012)

On August 21st, Producer 2K revealed during Gamescom 2013 a new expansion/DLC for XCOM: Enemy Unknown called XCOM: Enemy Within. It will be available also for consoles and has a release date of November 12th, 2013 on the US, and November 15th internationally. There are more surprises coming according to the producers other than what's revealed so far.

A list of details of was revealed on August 21st on this video, along with these articles from Polygon and IGN:

PC/Console Versions

The PC version will cost 30$ and the Console 40$.
The PC will be sold as traditional DLC through Steam but the console owners will have to buy the Commander's Edition, consisting of Enemy Unknown, plus the Elite Soldier Pack, Slingshot and Enemy Within DLCs.
Currently Firaxis has no plans to launch Enemy Within for iOS, due to the already large file size of the original game for the device.

The Meld

The Meld is a new resource that will be present in containers on several maps (Abductions and UFOs).
Meld is composed of organic nanomachines.
The 2 Meld canisters need to be captured during a certain timeframe, otherwise they will self-destruct forcing a player to move swiftly to find and capture them. 1 canister will be located on a location easy to reach, another in a hard location. The canisters can be hidden to force a player to look for them.
- Additionally, the Canisters are reported to spawn once per applicable map per playthrough: there will only be a certain number of Canisters in a given campaign.
When Meld canisters self-destruct they will explode and cause damage to any player units nearby. [1]
When you kill all aliens on a mission you'll automatically recover any unexploded Meld containers, including still hidden ones, according to a Rock, Paper, Shotgun interview.
The Meld can be used to create either Mech Troopers (a new class) or to genetically increase any class abilities (including Mechs).
- In consideration of the limited number of Canisters, it's also been revealed that this leads to a choice of quality vs quantity for how you will spend the upgrades.
The Meld also brings 2 new base facilities: Genetics Labs and Cybernetics Labs. There's also a mention about a "single, very low-cost new tech to power both" on this interview. The Cybernetics Labs count as workshops for adjacency bonus.
Overuse of Meld may 'corrupt' your soldiers.
There will be a small tutorial to show how Meld works.

Gene Mods

Alien Autopsies will reveal genetic modifications ("Gene Mods") for your soldiers.
Gene Mods can apply to the following categories: Brain, Eyes, Chest, Skin, Legs, with 2 options available for each (and that can be swapped at a cost)
Brain: Neural Feedback (damages an alien attempting mind control but doesn't reduce its chance) or Neural Damping (soldier immune to panic, becomes unconscious if enemy manages to MC it).
Legs: Muscle Fiber (a soldier can climb walls without a Skeleton Suit) or Adaptive Bone Marrow (limited health regeneration for wounded soldiers).
Chest: a double heart ability to protect your soldiers from death.
Skin: Bioelectric Skin (allows a player to detect unseen aliens) and a type of stealth skin that turns your soldiers invisible.
Eyes: Hyper Reactive Pupils: gives +10 Aim to a 2nd shot, if the 1st misses.
Adrenal Neurosympathy: soldier gives bonus to rest of squad when he kills an alien.
Modded soldiers have a different appearance than regular soldiers, with less armor being shown.

Gene Mods
Modded Soldier

Mech Trooper

Mech Troopers can be created from any soldier above rookie rank, but they'll lose all abilities they previously had. For example, if player sacrifices a sergeant-ranked soldier, he/she would get sergeant-ranked MEC, along with the ability to purchase skills from the first three ranks.
According to the developers Mechs are very expensive to purchase.
Mech Troopers have the Mechanized Exoskeleton Cybersuit or MEC Suit which can be damaged and need to be repaired.
There are 3 tiers of MEC Suits and each allows for a choice between two tactical subsystems: the Warden has the Kinetic Strike Module, which allows to punch aliens in the face, dealing 12 damage points, or the Flamethrower (deals 6 damage on an arc); the Sentinel has the Grenade Launcher or Restorative Mist; and the Paladin tier has the Proximity Mines (which are smart not to detonate upon friendly units) or the Electropulse (electric charge that can disable enemy robotic units).
Mech Weapons: Minigun (starting weapon), Railgun and an energy weapon.
Mech's skill tree is divided into two categories: Fire Support (offense) and Survivability (defense)
Mech Trooper abilities are called base augments. Those include: Shock-Absorbent Armor (starting ability); Collateral Damage (allows to target terrain), a high powered firing attack that uses a full clip of ammo, Damage Control (which reduces the damage taken for one turn), and One For All where the Mech acts as cover for any squad mates behind it. Another ability mentioned is Jump Jets and Expanded Storage.
Mech Troopers "remember" the original soldier class and "inherit" a passive ability from them. For example Sniper-turned-Mech gains the Platform Stability perk, which grants it additional damage for a turn if it doesn't move. Former Support Mechs have a Distortion Field that increases defense on nearby squad mates.
Mechs can't use Cover (and they can't be flanked). This mean that they probably have the Hardened ability present on SHIVs and alien robotic units that gives increased protection against critical hits.
As per GameSpot article, Mech Troopers get monotone, Robocop-esque voice.

The Mech Trooper
The MEC Suit
Burn baby burn

New Aliens

The Seeker

Aliens also have two confirmed new classes, including the Mechtoid (Sectoid Mech version). The Mechtoid can fire twice in a turn and Sectoids can mind merge with it and give it a 6 damage reducing shield [2](but it won't die if the merger is killed). Mechtoids are immune to the effects of the new Poison Grenade and to the Flamethrower. Mechtoids replace Sectoids as the game progresses.
The 2nd new alien is the Seeker and has been revealed on his video [3] to be some sort of a biomechanic floating squid, somewhat similar in concept to TFTD's Tentaculat, that can become invisible to your soldier's eyes.
- Seekers have a new AI, designed as hit and run units that will use their stealth to approach lone soldiers (such as Snipers with Squad Sight) and attack them through a melee attack of strangulation. The victim will not die immediately but will take damage each turn until the Seeker is killed or releases its victim. After a soldier is released from strangulation it will have to catch its breath. Seekers also have a ranged plasma attack.
- Seekers are immune to the Flamethrower and arrive during April. They can be detected through the Battle Scanner and the Biometric Skin mod.
The first Mech alien was based on the Muton but the idea was dropped because of the alien's already big size.
According to this interview, Mutons and Sectopods will be also improved to balance the addition of the new features.
Gupta confirmed on this video interview [4] that 'a bunch of enemies will be received'.
Another report mentions 'several' new aliens. Wait and see...

Class and Weapons Changes

At least three new grenades: Needle Grenade (after researching Chryssalid Autopsy), with a big blast radius but doesn't affect enemies in cover; Stealth Grenade, provides allies with a invisibility paint, a Flashbang Grenade and a Gas Grenade.
New item: Mimic Beacon - can be thrown on the map into a square within the player's moving range. The mimic beacon will send out a signal that tricks aliens into thinking it's human, drawing them towards it and away from any nearby player units. Robot and psi enemies aren't fooled.
Reaper Rounds gives ballistic weapon a bonus to Critical Hit chance.
Assault's Close and Personal now gives a free shot when within 4 tiles of an enemy.
- You can run up to an enemy shoot it and then run back into cover.
Support's Covering Fire now fires before the enemy uses its weapon.
Sniper's Snap Shot aim penalty reduced from -20 to -10.
Sniper's Squad Sight only gives Critical Damage if Headshot is used.
Support's Deep Pockets now gives +1 charge to Grenades, Arc Thrower, Medikits or any other item with limited uses.
Tactical Rigging - Foundry upgrade that gives all classes an additional item slot.
Respirator Implant: new item to protect from Seeker strangulation.
Soldier progression has been delayed to balance the new Mechs and GMods.
HEAT Ammo doesn't double damage, but only adds 50% now.
Sentinel Module (Foundry project) gives the SHIV an ability Close Combat Specialist.
SHIVs can also be further upgraded, based on the new Mech abilities.

Gameplay Changes

New Second Wave options.
- Training Roulette - After a soldier is assigned to a class and receives the starting ability it will choose afterwards from perks from other classes [5]. This applies only to abilities that are not dependent of class weapons, such as Rockets.
- Save Scum – Saving a game will reset the RNG seed for the shots.
- Aiming Angles – the closer you get to flank an alien, the more its cover bonus declines (works also against your soldiers).
- Itchy Trigger Tentacle - non-cover aliens have a 50% chance of firing or going into Overwatch when discovered.
Non-Cover Aliens (Sectopod, Cyberdisc, Mechtoid) will go on Overwatch when they are activated after the soldier who discovered them has finished moving.
Critical bonus of stealth attacks reduced from 100 to 30, likely to balance out the stealth grenade.
Panicked soldiers will have chance to hit decreased, so friendly fire will be less deadly.
Difficulty in Classic and Impossible modes will be slightly increased, according to the developers. Tech costs have been increased ddramatically for both levels.
Several issues have been fixed, including Line of Sight.
- Teleporting of aliens has been removed.
- Flanking bugs have been fixed.
Alien Starting Areas have been changed. On the Pax Prime video [6] the Outsider starts outside a Small Scout instead of inside.
New mission types hinted by the developers (to be revealed later).

Interface Changes

There will be an inventory button to strip away all items from soldiers not in the squad lineup ("Make items available").
You can now personalize your soldier's voice according to its nationality (French soldiers can speak in French or any other language) using the available audio packs for the current language editions (following languages were confirmed: Spanish, German, Italian, French, Polish, Russian). The personalization applies to soldiers nicknames as well. The game footage released has nicknames such as Tirailleur (French for skirmisher), Unruhe (German for Restless) and Maestro (Italian for Master).
There's an option to shut up Drs. Vahlen and Shen - Reduced Beginner VO.
Memorial (place where you see fallen soldiers) will have additional information shown: cause of death.
A.C. Clarke's quote at the beginning of the game has been replaced with one from the futurist Buckminster Fuller: Those who play with the devil’s toys will be brought by degrees to wield his sword.
Medals for soldiers will be introduced.
Color customization options now apply also to weapons being carried by the soldier, as well as armor.
- Mech Troopers have a new customization feature allowing to choose between tiger stripes, flames or spiral decorations.
- More helmets, armor decos and colors.

New Maps

Maps: some maps have been "tweaked", some to address exploding car issues and to add Meld containers.
The old and new maps will be mixed together but the game is set so that you'll see more of the new maps during a gameplay.
There's two new map settings that deal with crashed UFOs in a destroyed city and in a Farm. There's also a Dam map, most from the cancelled Progeny DLC (which will be included with EW).
According to this Polish article, there will be 47 new maps.
Snow covered maps are clearly visible on the Polygon video.
Other maps confirmed by Ananda Gupta during Pax Prime include a bank, ice cream shop, road house, coffee shop and a new truck stop.

Modding

In one of the interviews there's a mention by the lead designer that they moved more settings to the game's .INI files rather than the Unreal script to help modders.

Multiplayer

You can now edit your squad on offline mode.
8 more maps for Multiplayer.

Grenades/Rockets and XCOM Rookies cost have been reduced, because they're considered overpriced.

Progeny

In this interview [7], EW's lead designer, Ananda Gupta has confirmed that Progeny, the 2nd planned (and cancelled) DLC, will be included with Enemy Within.
Gupta also said that the narrative has suffered changes to adapt it more with the EW expansion.

Unrevealed Features

Enemy Within will have Easter Eggs related to The Bureau: XCOM Declassified.
The Enemy Within trailer now has a new version that includes a brief glimpse of a new symbol. Wait and see...

Quoting from Rock, Paper Shotgun article:

RPS: The teaser does mention a new threat. It’s not talking about Mechtoids, right?
Gupta: I’m really looking forward to talking about that (laughs).
RPS: Everybody has asked you this. Base invasions. I’m not even wording it as a question.
Gupta: I love the way you’re thinking.