The game Army of Two and Army of Two: the 40th day were both created by EA Montreal. Both are a third person shooter and both are focused around a core element of co-op play. Army of two is based on the military and you play as a a soldier that has a partner which is AI or two player controlled. The story is about a series of missions that begin and end but then you are pulled back into the missions years later. It focuses on government and military fighting criminal masterminds and terrorists and that you are the only team that can stop them.

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The main feature is how the Co-op system works with build in objects in the game that need the use of two players. It also uses two players to change and effect game play. The first co-op game play changer is how 'Aggro' is used in the game. 'Aggro' is how noticeable you are to enemies so the higher it builds the more you are targeted but this takes away from the secondary player or Ai creating them almost invisible at times. Guns and weapons produce more 'Aggro' than others meaning a players can choose who is playing stealthy and who wants to be targeted.
Another game changing feature in the game is the back to back mode that is build into levels. Back to back is a game feature that places both players back to back allowing them to cover a whole 360 spread of enemies. At these points the enemies would spawn and fight you from behind scenery or in open area. This mode is also in slow mo making the experience seem that its on a much quick level in real life. I like this feature but its only built in so only at certain points on levels it happens.
Healing you partner is a good feature for when the players health does down he ends up on the floor for a period of time that allows the other player to come and heal you. It doesn't stop there, if you have gone down then this means that the area you are in is under fire so healing you there isn't the best place, this is why in the game you can pull you team mate to safety. I like this feature because it shows how helpful another player it is but it also does mean players can just throw there bodies into fights and get dragged out to repeat it again.
Players can swap weapons which i think should be a key feature for all FPS games as a lot of games you have to either find weapons or savaged of killed players or enemies meaning you have to get together over a weapon to drop it and then have a friend come to you to pick it up. Its slow and not helpful in a difficult situation.
the start of a level you parachute into the map meaning that one player can control the direction you travel with the other able to shoot out enemies in the landing zone. I think this worked but it wasn't fair on the players who got to shoot and direct as they cant choose. The idea of using vehicles that require two people could be a interesting idea.
Players can also help each others climb walls and buildings by players boosting each other up but a lot of the time you only use this feature when the creators what you two with walls in the way of the next part, it gets annoying when you could make another path around but there is a wall just so they can make you use that feature.
All these Co-op features are interesting and are relevance to me as i wish to produce ways that players can interact together in two player but i don't want to have built in elements. I want players to be able to play the game without a constant AI player and not to always rely on a secondary player because i feel it ruins the pick up and play element i want the game to stand out on the single player element with the Co-op being a good added bonus meaning players will want to play the game first then return multiple times with friends and online players to explore allowing different experience and more replayablity.
features i want to include are similar to Army of Two like how players control different elements of vehicles and swapping weapons but i want also get a feel of team work against opponents like being able to both interact and take down a enemy without having to just whittle down its health by shooting at it.