The title of the thread doesn't appear completly... holy shit.... It was

Coop Mode / What Are The Good Features To Implement ?

Anyway, my humble opinion is it's possible to change few things in order to have fun in Coop mode. First: we can boost monsters health, in order to increase difficulty during fights. There was also a suggestion about ammo/weapon sharing, and I'm not sure it will increase fun..
Concerning mapping, for sure dedicated maps are required: 2 starts, more monsters, more ammos, more fights, more fun... 

i was thinking of having one 'ammo pool' where every player shares from that same pool.

the limits would be a lot higher, like 1000 nails, 500 shells, 500 rockets, etc... basically normal ammo * 5.

but i dunno how this would work out in practice...
on the one hand, it makes ammo sharing simple, but could also lead to problems, like someone wasting all the ammo or something like that. O_o

and what would be prefereable, spawning a monster in twice, right after the first one is kill, or simply upping it's health by 2x?
i'd be more for spawning two in because then you still have a chance to take on any given monster solo if your partner is missing, but if they have 2x health, it makes it more likely you'll die. 

what would be prefereable, spawning a monster in twice, right after the first one is kill, or simply upping it's health by 2x?

No offense, but IMO that's a bit like asking "Which would you prefer, a nail through your left foot or a nail through your right foot?" 

I don't like the idea of changing the monster count because it's not addressing the amount of room the players and monsters have to navigate. And I don't like changing the monsters' health because by now players are accustomed to monsters dying after a certain amount of damage. I don't like the idea about changing the ammo packs for a similar reason.

I think the best option for coop play is to have smart design on the code side (keys and weapons that stay like in DM, all players start a level or respawn with ammo and health greater than a set minimum amount, and maybe when a weapon is picked up by a player it goes into a "pool" of weapons that all players have when they respawn) and levels that are designed to play well in coop (plenty of area to navigate, a higher difficulty rating that normal SP maps, tasks that have distinct approaches depending on the number of players, non-linearity, a vague hub-like design). It's issues on the latter that I wanted people to comment on for my own purposes, but the former is an interesting topic as well.

Now one thing that I think would be interesting is to have a specific coop mode where monsters respawn a la Doom's Nightmare mode. The problem of ammo, armor, and health shortage could be solved by respawning those items. And a monster that respawns would be stripped of its "target" key so it doesn't accidentally retrigger an event that it shouldn't. And then on top of the coop mode, players could set the skill setting to determine the quantity and difficulty of the monsters in the map. (i.e. the coop setting and skill setting are set independantly so you can monster and ammo respawning on lower skill settings.) 

(i.e. the coop setting and skill setting are set independantly so you can have monster and ammo respawning on lower skill settings.) 

spawning a monster in twice

Your idea was basically what I just posted, wasn't it? The only difference being that I suggested the monsters respawn infinitely--with perhaps intervals of 20 seconds--and that the items also respawn. 

I like the idea of a map being designed that it could be done by a single player, but using 2 players will get the job done quicker.

Let's say the start of the map offers 3 paths. 1 path is blocked, requiring 2 buttons to be pressed. Each button is down the other 2 paths which lead back to the start area. A single player will have to deal with twice as much play, and fighting. 2 players could split up, perhaps each getting a differant set of weapons, meeting up at the start again to tackle the large battle that through the locked door. Simple and effective coop design. 

Anyone ever tried System Shock 2 co-op? Full of hilarious moments. People you play with are far more dangerous than anything the computer throws at you.

"Step aside, I can't fire"
"What. You're standing in front of me."
"No. You."
"We both can't be standing in front of eachothers!"

At this point the security bot molested both of us to pieces. So yeah, have a proper netcode.