My assumption was that each player levels up only for a match (like League of Legends or a DOTA) and not a persistent thing. If that's not the case, then I'm also stumped.

Have you ever played Castle Crashers? The end of each chapter sees a princess rescued...but only one of the four players gets a kiss. To secure that kiss, the players are pitted against one another immediately. Some characters (Red Knight with his lightning and Green Knight with his poison) are far better suited for this than all of the other characters and very often come out on top and get the kiss. Now, the 'kiss' is meaningless...nothing more than brief bragging rights and a good laugh. But if the game was centered on this, then suddenly there'd be a lot of legitimate complaints around the VS balance.

Not to mention, this game idea seems very hard to justify; why would a team of individuals who need one another to get to the helicopter suddenly decide to turn on one another? People wouldn't do this "just for the experience points". Don't forget that XP and levels are an abstraction. Again, this lends toward Munchkin in that it seems to be tongue and cheek.

Picture this:

2D Top Down Retro 4-player Team Based (Sorta)

Premise - Limited Story
Group sent out to retrieve something from zombie infested wasteland.

Objective - Enter. Locate objective. Retrieve intact.

Mechanics - Team work is key but when fast leveling and increasing power builds your characters you can kill anyone of your teammates in order to get more experience. Earlier kills gains more experience. Kills near extraction give little to no experience if entire team survives.

Heavy
Medic
Stealth
Engineer

Heavy - More Hp and Dm. Slows team down and easily overcome alone.

Medic - Can heal the team and himself. No reason to live other than heal.

Stealth - Stealth field. Invisible to zombie and Team. Broken with gun fire. Knife and stay invisible. Invisibility lasts longer with more team members. Alone doesn't work.

Engineer - Fairly useless offensively. Can hack turrets (Turrets loyal to engi if team attack). Open shortcuts. Can repair objective.

I can see where you're going with this, encouraging teamwork.

It seems like any team would require a Medic of any sort, basically running into the old "lfg healer" issue. If you've ever played Killing Floor, they had a pretty good healing mechanic there. Basically everyone could heal themselves, but a) it means changing weapons for a short time to pull out med syringe, thus making it a non-viable tactic if alone, and b) self-healing is far less effective than healing another player. There are no fixed classes in that game, though each player got to choose a 'perk', one of which was healer - which only gave increased healing without any other drawbacks (but no additional benefits)

I can see the role for each, except the Stealth. While he can benefit from more players, it doesn't seem like he'd contribute anything to the team, while all other classes do something more .. unique that no other role can fulfill. This reminds me a little of the damage dealing classes in some rpg/fps games, which are usually the least team-oriented players, since they only benefit from their team and don't contribute much (and often seem to just run off expecting the whole support crew to run behind them)

I think it's important that every player not only relies on their team, but has something to contribute as well.

Going a slightly different way, without 'classes' - the way L4D dealt with this was beautiful. Every player was equal, but self-healing was slow (just as Killing Floor's), and the most unique part was that certain zombies would immobilize individual players, requiring the help of another.

Well, I guess this doesn't solve the core issue here, which is that all these games encourage cooperation only, and thus somewhat harshly punish uncooperative players. That also may be an issue with the classes you proposed, as if a medic decides to go backstabbing, he's left alone without much of a chance.

I can't imagine how a non-coop coop game would work without any quick respawns, and low penalties for death, but that doesn't mean it can't be done

To address xp and leveling: the game is played game by game (4 matched per game) for fun. No overarching use for xp. Reset ever game. I guess I always imagined playing this with my friends so, like L4D, you can communicate but also mock and plea. The idea of classes seemed to fit because most had equal pros and cons (you're right about stealth though) but this also makes players act mire tactically in when to betray someone. Say a stealth attacks a heavy. The medic and engi would have to quickly chose sides. Weighing up options. Is the stealth reckless? Can we survive without the heavy? Overall there is a low penalty for death. But this means quick ability to retaliate. Holding a grud can payoff in some cases.

Some of the starwars games had side games that were Co-op. Basically you and a bunch of friends were put in an areana to face unending waves of enemies.

I could see a mechanic where players are given "tricks". basically abilities that modify the game whenever a player activates one: Enemies counter tracking missiles, players shields slowly drained(soft time limit), and other similar mechanics that affect ALL players.

So all players are rewarded for surviving and similar. However if a player dies when one of your tricks are active you get extra bonuses.

____

Could see this brief mechanic going well with a horror game. You know you have allies who want larger rewards by advancing further, but at the same time you know those same allies want you dead from time to time with expected help just not coming.

Some of the starwars games had side games that were Co-op. Basically you and a bunch of friends were put in an areana to face unending waves of enemies.

I could see a mechanic where players are given "tricks". basically abilities that modify the game whenever a player activates one: Enemies counter tracking missiles, players shields slowly drained(soft time limit), and other similar mechanics that affect ALL players.

So all players are rewarded for surviving and similar. However if a player dies when one of your tricks are active you get extra bonuses.

____

Could see this brief mechanic going well with a horror game. You know you have allies who want larger rewards by advancing further, but at the same time you know those same allies want you dead from time to time with expected help just not coming.

Was that a really thing or something you came up with?