How hard is to balance a level for coop?

I've never played Doom coop, although I've watched some videos, but I'm considering balancing Exomoon a bit for coop. A proper balancing would definitely take a lot of time and effort, but what about some "general" balancing (like a mapper would do for HMP and HNTR without too much thought)? Does it needs to be like a difficult above UV (with more monsters and therefore ammo/health pickups?)

Thanks a lot for the help

Most well known releases -- if they make any substantial changes at all -- tend to go the route of conveniences for MP, rather than the MP-things-as-UV+ / -solo-net one. A lot of changes have 'avoiding nuisances in MP' in mind. Some useful tips:

- Above all, think about all your lock-ins and spontaneous barrier erections (new official phrase) and make sure a player dying past one won't break the map. This applies regardless of how (in)extensive MP-only thing placement will be.

- Multiply (3x or 4x) essential pickups (e.g., necessary radsuits, map-starter backpacks or armors) so that everyone gets one if playing without item respawn, and mandatory waiting is eradicated if playing with.   

- Armor and 'big' health pickups for fights or at other key junctures: a case-by-case basis taking into account how many players will likely do a fight together, and if all players involved will need that pickup. A single (SP) mega can become four in MP at one extreme, it can be bolstered with a couple (MP) soulspheres instead, it can be left entirely unchanged at the other extreme. Incidental -- peripheral + less important -- stuff is often left unchanged. It depends. Extensive secret area? Yeah probably add duplicates. Tiny alcove secret? No need to bother. 

- Ammo: favor dispersal and arrangements not involving stacking, allowing multiple people to get ammo smoothly. Say your SP start is a SSG with two shellboxes, you want MP to have additional shells. Weapon pickups after the first of a type don't register as ammo with some settings, which is worth keeping in mind if hitscanner corpses are a big source of a map's ammo.  
- Extra monsters: a smaller number of significant MP-only buffs can go far; there's no need to bolster every group. Look for spots where enjoyable groupwork might shine -- extra fodder or meat for larger or oft-revisited areas, additional threats for important fights. One classic/iwad trope is tossing in a roaming cyb (sometimes mastermind) in a big important area. A bit lazy but might be what you're looking for in a set like Exomoon. 

Thanks a lot for the answer, @rdwpa! I believe there's not any one-way passage which might break the maps that way, but I'll have to analyze each one of them and make sure everything is alright... (the original moonblood had several places which would prevent additional players from progressing)

- If I choose to multiply a essential pick-up, do you think that they need to be close to each other (4 soulspheres in a row, for example), or the mapper can be free to put the duplicates in a bit more distant areas?

- By dispersing ammo, you mean by adding them in areas where originally there wasn't any in SP?

- The "filler" placements I'll keep them the way they are (as they would just delay players from progressing), so I'll have to focus on the key areas. In a non-linear map, should I expect each player to go into a different way or just expect them to be together? Or should I judge that depending on the map layout style? For example, some sort of city open field map: expect players to spread vs "tricks and traps" style map: everyone usually stay together.

Testing the map on solonet will give a rough idea of what adjustments need making. Often you'll need some additional ammo, and teleport or other short-cuts to access lock-ins. Some mappers recommend placing multiplayer only chainguns and chainsaws near the map start, as these weapons work well for coop.

One thing I personally do when I'm streamlining a map for single player, is to set encounters that feel like filler to multiplayer only.

I'm pretty sure almost everyone plays coop with respawning items nowadays, so I don't recommend going crazy with multiplying the powerups. Having a free soulsphere immediately available for everyone is usually way too nice, it can reduce the challenge to disappointingly low levels. The radiation suits are, of course, an exception: there are never too many of these. Multiplying green armors is okay.

If there are no chaingun/shotgun pickups in the level and the player is expected to get them from chaingunners/shotgunners, you should add some multiplayer-only ones to make sure that everyone can get these important weapons. Try to put them in noticeable spots too.

Please please please don't overuse the "multiplayer cyberdemon in a big area" trope. Yeah, it's fun, but it can also get old really quickly. I'd say using it once in about four-five maps is okay.

One of the most exciting things you can do is to put the multiplayer starts in slightly (or even vastly) different locations. I love the adventurous feeling when everyone appears in different spots and we have to look for each other.

2 hours ago, Deadwing said:

If I choose to multiply a essential pick-up, do you think that they need to be close to each other...

No! Some bastards just don't care and they will refuse to carefully maneuver between the powerups.

2 hours ago, Deadwing said:

In a non-linear map, should I expect each player to go into a different way or just expect them to be together?

I feel like it depends a lot on the people. If I'm playing with close friends we will often try to work together at all times, but if I'm with strangers I will usually try to get as much action for myself as possible. :)

It depends? Vanilla Doom is pretty tough: There's only the Multiplayer flag (so any Deathmatch additions will also be in co-op, and vice-versa) and no way of changing how many of a given item spawns based on player count. It's much easier in ports that allow you to flag things for specific modes and number of players, however most co-op servers use respawning items that wreck any semblance of balance, anyway. (Co-op is much better the fewer players present, IMO.)

Your multiple pickups should be far enough apart that one player cannot accidentally take more than his/her share. We never played coop with respawning items.

Also, some coop players are not very coop friendly, and take great pleasure in snatching up all the goodies. So it doesn't hurt to scatter some extra pickups around.

Thanks for thinking about coop - I'm always looking for good coop maps!

On 8/21/2018 at 11:51 PM, rdwpa said:

- Above all, think about all your lock-ins and spontaneous barrier erections (new official phrase) and make sure a player dying past one won't break the map. This applies regardless of how (in)extensive MP-only thing placement will be.

Conversely, though less urgently, think about how multiple players can break your lock-ins.

For instance, if you have a door that closes behind player 1, but it can be re-opened by player 2 from outside, this can dramatically change an encounter's challenge / fun.  In many cases it might be fine, but sometimes you might want to complicate things for co-op gamers (e.g. spawn in some additional co-op only monsters outside the original trap zone, so that retreating to safety isn't as easy).