What's on my mind is this:
-Each player (max 4) gets to choose 1-2 exos (2 player: 2-2, 3 player: 1-1-2, 4 player 1-1-1-1).
-Only the host's pilot skill is active; other players are "guests".
-All players vote on a mission; majority rule. If there is a dispute (equal votes on different missions), whatever the host has voted for will be chosen. This can be disabled, and host can be granted "power" over mission choice before loading a saved game / starting a new one.
-EnemyBots gets buffs, some new co-op stuff are introduced (discussed/explained below *1).
-"Guest" status may also expand to inventory management, exo customization and shopping too. Host can decide what guests can / cannot do while loading a save / starting a new game.
-All Exos that each player control are spawned at the same time, game requires players to commit their actions (gameplay example below *2).
-Host can choose between shared and private inventory which cannot be changed after a game is started. If private inventory is chosen, all players get full single-player rewards, can access shop, and customize their exos. No trading between players (?). Individual loot follows a round-robin rule to get distributed. If shared inventory is chosen, it is like single player, but with one difference; more rewards are gained at the end of the mission, depending on how many people are playing the game, customization and shopping depends on the game setting.
*1 Co-op pitfalls, suggested mechanics, and stuff:
There are possible problems with the enemies scaling with player count. Increasing the HP and damage of the enemy is not really the answer to balance the game for a co-op experience. An example game: Borderlands. I really don't like Borderlands, because it does whatever it can to discourage co-op. The kill skills, which activate after you kill an enemy are a direct contradiction to the nature of co-op. Also, enemies becoming incredible hulks with each player; becoming "Bullet sponges" is another pitfall to be avoided. Or, enemies suddenly destroying you since you are trying to play the game with more people.
While they should become tougher and their numbers should increase, it shouldn't be too much. Other mechanics that encourage co-op should be explored in addition to those. For example, a bot that only appears when you play in multiplayer (Co-opBot lets call it for now) is a way to do it.
Example Co-Op mechanics:
-Exo disable: Players have certain amount of repair kits (just 1-2); if an exo gets hit by a shot that'd kill it in single-player, it instead gets disabled, before going critical and "die" in a few turns. If a player reaches the disabled exo, it gets re-activated with a portion of HP, and that player's repair kit is expended. Like in many co-op games, you are helpless on your own, and you wait for someone to get to your aid (L4D, L4D2, Warframe, etc...)
-Exo link: Active exos could provide bonuses to other players' active exos, a Sniper exo could increase the range of the other exos, etc... IF both exos are within close range (5-7 max). While enemies get a boost, players will also get a boost as long as they stay together. Sensor data is also shared when exos are linked to each other.
-Abilities (below), visual cues to plan a turn (mentioned above; pinging a tile/exo, seeing which player is attacking who, and dealing how much damage beforehand, etc...)
Example Co-opBot X abilities:
-Deflective Shield: Deflects any number of shots or damage unless gets focus-fired. Lets say me and a friend are playing, and we faced a bot with this ability. The bot has a deflective shield. Unless we both fire at the same target, it will deflect all but one point of the damage. This means, we need to stay together to deal with these enemies (ignore the sentries & hacked bots for now). AoE weapons are a different story, deflective shields deflect a big portion of the AoE damage, but does not reduce it to one point.
However, there are sentries and hacked bots that interfere with this "idea"; one way to deal with it is simply ignoring them altogether when it comes to dealing with deflective stuff. Another way to deal with this is, giving an ability to exos to break this deflective shields, although only when fired by multiple exos. When this ability is used on the same target on the same turn, deflective shields will be broken and the bot in question can be damaged on the next turn. The deflective shield recharges on the turn after that if the bot didn't take any damage.
Well, as the name suggests, bots having this ability will not take any damage from one side (most possibly front), or just take a portion of it. But this idea is inferior to deflective shield, as it will slow the game down, and may force the players to spend a lot of time dealing with this. Another approach to hardened armor; Some (all?) exos could have a target-painter, a whistle-like infinite amount ability to mark weak spots of the enemies with hardened armor. Other exos could fire at this target, dealing full damage. This 2nd approach promotes co-op even more, similar to deflective shield, but treated in a different way.
Similar to EMPBot; but it has a different mechanic. It is a very short range ability. An overloaded exo will take 5-10% damage each turn, regardless of damage reduction, and cannot regen. The bot having this ability will keep using this ability till its target is disabled. An overloaded exo will not be targetted by other enemies. It is a temporary disable so to speak. Think of special infected from L4D series. Unless the bot having this ability is destroyed, the overloaded exo will get disabled, and a repair kit will be needed to reactivate the exo. If the aggressor has been dealt with, overloaded exo will reboot on next turn. Overload will not affect the last remaining player; if all other exos are overloaded, or disabled, or exploded, the last player will not get this ability cast upon (otherwise, it'd be a mission over each time you got "unlucky").
*2 Gameplay Example:
Lets say I'm playing with a friend. First, I choose a pilot. We each pick 2 exos, and we vote on a mission, and mission begins. 2 bots spawn next to each other, and we play at the same time; we do something, then we commit our actions, and then watch the results. This has one big problem: game will be slowed down. Immensely. So, how can this be avoided? If there are no alerted enemies, no skills currently being active (stealth) no commit would be required. When there is a fighting going on, when I click on an enemy, my exo will point a green laser on that enemy, and a damage number will be shown over its head, so my friend can decide whom to shoot, or not without having to go through chat fast(er). Also, I can click a button to ping a tile or mark an enemy for my friend with ease. Any AoE effects that'll happen will tint the tiles slightly to red to warn me / my friend, so we can avoid it. Whenever a turn is committed, our actions are the first to be executed (like in single-player), followed by AoE and enemy actions.
I selected shared inventory, and since we've won a mission in co-op, we got more loots than usual. The same mark button can be used on inventory/shopping panel to "mark" items, my friend cannot unequip items on my exo, but he can mark them to tell me if he wants to try it or not. If I was playing with someone else that I don't know really well, I would've set the lobby with private inventory enabled, but this is not the case. And game goes on like this...
----Excuse my english, haven't slept, did a lot of re-writing, and forgot some stuff while writing. Any numbers are for example.