My studio (unnamed, we're calling ourselves the "Subroutine Samurai" for the moment) is still brainstorming on new stuff while we develop our current project. Coop games seem to be golden to us, and we really want to further the genre, but we've run into alot of issues on building immersive games in that matter. Namely, suspension of disbelief. Adding void chat, and blah-blah-blah makes it hard. In a quest for a solution/strategy, I need answers to some questions: What is the usual number of players involved in your games that you play "together"? 2?, 3?, 4? How many consitutes a lan party? Do you play online, or on a local network only? Do you often use dedicated servers or listen servers? when playing coop games, do you always choose people you know? or will you take on a stranger as a "partner"? (This may apply to MMOs) Do you have issues with cheating in cooperative play? How long do your games need to be? Should they be "completable" in one sitting? saves? What about dynamic joining/exiting a game? Should we take special care to ensure that the game accomodates this? Positive identification and roleplaying (this is important beyond 2 players). Should players be allowed to don their own names, or whipped into character? Is there a point where this is bad? Note that this is in relation to just about any coop game. Some of the stuff we're designing is scalable - much like serious sam. Others are pretty fixed to 1-4 players without too much hope of scaling.

I don't really play too many coop games. But i'll answer some of your questions anyway.

How many players depends on the game. I think Halo would have worked well with up to 4, but more than that would be a crowd.

I've been to lan parties with anything from 4 to 4000 people. But i rarely communicate much with more than 10 of them, the other 3900 are just for leeching files :P

I don't really think you need dedicated servers for a game with just a few players. It just makes more sense to have one of the players host the game. Again, this depends on the rest of the gamedesign. So does dynamic join\exit.

Coop is best with friends. But playing CS, i can follow a complete stranger and get some pretty nice tactics going anyway. You should design for both if possible. Don't require too much communication.

Games should definitly be completable in one sitting. Forget about saves. I feel it's best to divide the game into several chapters if you need to, and let the players choose which one to play. Go back to chapter selection after the chapter has ended. If people don't want to play the correct sequence and miss out on parts of the story, so be it.

Let players use their own names UNLESS you're trying to camouflage the fact that they're playing with other people. In that case, playing with random people online is the best i guess. Other players would just seem like NPCs with totally awsome AI. I think that would be cool, kinda a cross between multiplayer and singleplayer.

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What is the usual number of players involved in your games that you play "together"? 2?, 3?, 4?

Depending on the type, anywhere between 4 to greater than 10.

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How many consitutes a lan party?

I don't generally have many lan parties, but i would probably say at least 4, any more than that is icing.

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Do you play online, or on a local network only?

Online

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Do you often use dedicated servers or listen servers?

A combination of the two, it depends on the game being played.

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when playing coop games, do you always choose people you know? or will you take on a stranger as a "partner"? (This may apply to MMOs)

Usually strangers, in MMO's i usually work together with strangers and may over time develope a friendly relationship.

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Do you have issues with cheating in cooperative play?

Yes, extremely so.. I find cheating deprives the game of purpose. Cheating simply strips all the challenge out of the game, and its particularly bad when someone else does it, since that also spoils my efforts. Generally its not a good thing, Diablo 1 for instance was so infested with hackers it was utterly pointless to play online if you were trying to play 'legit'.

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How long do your games need to be? Should they be "completable" in one sitting? saves?

This depends heavily on the type of game. Planetside is a MMO with a persistent world, so even if i login or logout, there will be people carrying on the fight whether i'm there or not.
For things like saved games to continue later, this wouldn't neccessarily be a bad thing, but might be abit of a mood killer. Having a particular chapter lasting around 1 to 2 hours would be good, about the length of time of a good movie. The players can then continue with several chapters if they want, or get more if they choose.

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What about dynamic joining/exiting a game? Should we take special care to ensure that the game accomodates this?

This may be a very good option to have, some players may have poor connections. If they end up being disconnected for one reason or another, not being able to rejoin and having to start the game over again simply to include them would really suck. Equally, they may have some reason to leave the game (kids wreaking havoc, errands to run, etc).

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Positive identification and roleplaying (this is important beyond 2 players). Should players be allowed to don their own names, or whipped into character? Is there a point where this is bad?

I was never much for role-playing, images of people creating whole AlterEgo's just weirds me out. I'm alright with defining avatars and alias's, classes and the such, maybe even playing along a little. However i'm just not for roleplaying, this is my own opinion, although other people may really enjoy the option.