So you've played some multiplayer, perhaps you've battled online, or against friends in intense grudge matches. But your games are getting a bit stale, perhaps, you're searching for some spice to your XCOM:EW battling.

Well look no further!

This could not have been conceived without the help of Soapyjr.

Will be updated at any time we think of some stuff!

Big Boss is a tactical, protect-your-leader kind of game. The squad composition is identical for each player, and the game revolves around the protection of your Boss.
The Boss is a Sectoid Commander, and has great power due to his Greater Mind Merge ability. The minions can be interchangeable, either five Mechtoids, or five Sectoids, for a less intensive game. Keeping your minions buffed via the Boss is the way to win here, all while preventing the enemy from getting to your leader.

To prevent a lot of overpowering, the Boss is not permitted to use Mind Control, though other Psionic abilities and his regular attacks are fine.
As the Mechtoids can't climb ladders with their little legs, its only fair that the Sectoid Commander has the same disadvantage.
You could define win conditions as the Boss being defeated, but often when the Commander goes down, the Mechtoids don't have much against shielded, buffed Mechtoids and the odd Mindfray or Psi Panic.
A few potential scenarios could be having your Big Boss waaay at the back so he doesn't get bum rushed, or you could have your Boss in the meat so he can pop fresh shields on your Mechtoids, but he'll be a target due to the bright obvious streams of psionics.

This gamemode is all about raising your very own zombie horde from the corpses of your enemies.
Players begin with two chryssalids, and four frightened Rookies. Two of them have managed to scrape a grenade each from somewhere, but otherwise it's just base XCOM ordnance.
This ends up being a very tense, tactical game where you'll creep around maps, ears pricked for that tell-tale skitter of claws.
The rules for this game are simple, but it doesn't mean the match will be.

A potential scenario could be one team has all his cowardly Rookies bunched in a room with overwatch for chewing up enemies. The other team may decide to blow your walls up leaving you completely exposed to two angry and very hungry Chryssalids. This game mode can become punishing very quickly epecially when half your men are zombies, ready to burst.

(Whether this is a fact or a bug, grenades exploded on Chryssalid victims often negate the zombie effect, so bear that in mind if you lose a man to infestation - the game could still be yours)

Before the alien threat arose, humanity still possessed many flaws. Greed, arrogance and hatred filled the air and the nations of the world battled for supremacy of natural resources and land.

Presenting the Civil War gamemode! Xcom personnel battle it out using only ballistic weapons (Assault Rifle, Shotgun, Sniper Rifle, LMG and Pistols) and human equipment. Strictly no armour or Gene mods. For the most balanced game, use four Squaddies of each class, one Medic (Captain rank Support) and one rookie.

The items permitted are: Frags, Flashbangs, Nano-fiber Vests, Medikits, and Combat Stims.
This can be comfortably done under the 10000 point limit, and will force players to balance out their items.

You'll be forced to carefully manage your rocketeer, keeping them alive until they can use their explosives effectively. Your medic may be the focal point of your team, with that devastatingly powerful revive ability on anybody who may go down.

(Prompted by andrewp1017! Thanks commenters!)

[Suggested by VeryTallGnome! Thanks!]

Psi-copaths is a gamemode in which you battle for metal supremacy against your equally matched opponent. Players each have one Ethereal, two Secoid Commanders and the rest are max level psionic Xcom personnel (typically one each of Sniper, Support and Assault, since Heavy has no psionic multiplayer class :( poor guy!)

Using only Alien grenades, one Medikit and one Combat Stims, XCOM ballistic weaponary (This is a battle of brains, not brawn!) and no use of Mind Control (But in the end it's up to you and your opponent, to predetermine these rules beforehand!), you'll duke it out, dodging Rifts and ripping your enemy's mind from within! Gene mods, to use Lurker or Bastion is massively overpowered for this mode, so don't, but everything else is your own choice.

A potential scenario could be, you have a couple of chaps on your team that decide they really want to give the Ethereal a headache. Now your first guy gets lucky and hits his mindfray, The Ethereal's head is now throbbing, he's in no state to take more random headaches, but now he's open to your friendlies giving him all kinds of head problems and migraines.