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February 22, 2021

Transmisson 04 - Improving Co-op

Improving Co-op

This post will focus on the Co-op mode. It might be early to talk about it, but it's my favorite mode, and I want to share some thoughts about it. While many topics could be discussed, I will mainly focus on some ways how to expand the mode.


Co-op is a successful game mode in StarCraft II that focuses on 2-player PvE. Players can choose from a roster of 18 commanders which can be leveled up and customized. Every mission is highly scripted, standalone, and grants experience to players.

The name Co-op isn't very descriptive as it can mean other things – 2v2, 2vAI skirmish, or cooperative campaign. But for this article, I will be using it as described above.

Co-op's success was unexpected when Legacy of the Void was released, but now it stands as one of the most important modes alongside campaign and competitive. The purpose of this mode isn't to help players to move to the competitive multiplayer, but instead to provide a different experience to the playerbase that seeks it.

I have explored some ways to expand this mode, and I will include screenshots and commentary. All avenues to expand Co-op have their design challenges – some might fit into the future game, others might not. In my opinion, expanding Co-op from its current state would improve its longevity, engage more players, and add value to every single commander released.


This integration can help to make streaming more engaging for both streamers and viewers. My implementation was mainly a proof of concept and a fun project. It works on my arcade maps when using a twitch bot from my overlay app.

I've added two modes from which the streamer can choose:

  • Partial integration – gameplay is unaffected, viewers can send messages and join as units. Kill counts are counted for viewers per mission, in total, and are shown after the mission.
  • Full integration – viewers can affect the game by spawning units or waves, enabling or disabling mutators, and giving resources.
Viewers joining the game (no gameplay impact)

I found the partial mode to be good for viewer interaction. Full integration is harder to implement and balance properly. For that inspiration can be taken from Warhammer: Vermintide 2 which includes a twitch mode where viewers can add buffs or items to players, spawn enemy units or activate mutators. The Riftbreaker, an ARPG with base-building, lets viewers join as enemy mobs, and vote on upcoming events.

Those are just a few examples, but I believe integration can make streaming better for both players and viewers if done well. I don't know if it can fit in a competitive setting, but it's well suited for modes like Co-op.

survival mode

Survival or horde modes can be found in almost all action genres, and so it seems like an easy choice for Co-op.

I have tried creating my own for one of the Co-op maps. The purpose was to demonstrate how a survival mode could be implemented on top of a standard Co-op mission. In this mode, players have the option to continue playing the mission after the regular part of the map is completed. Additional attack waves will spawn and provide an increasingly difficult challenge until commanders inevitably fall. This is very much like the original campaign mission Last Stand where you have to lure as many hybrids as possible before the temple on Shakuras is overloaded.

The march of Kaboomers
Motherships using recall offensively

There were challenges for this mode in StarCraft II. The most obvious one is that the engine is pushed to its limits even in normal missions. While I tried to keep the performance at playable levels, I do hope the performance of Frost Giant's game will be better – unlocking more gameplay options.

Asymmetric 2v1

In this mode another player gains limited control over enemy units and provides an additional challenge for commanders. I tested the waters with my implementation on the Chain of Ascension mission. I thought it was an interesting idea and was curious how it would work out.

An important point is that this mode doesn't have to try to target 50–50% winrates. Trying to balance Co-op commanders in a competitive setting is futile, too restricting to their design, and Co-op players don't want the same type of challenge as competitive players do. Instead, the opposing player can spice up gameplay and add only a tiny bit of the competitive thrill. Even then I imagine this being tied to higher difficulties unless in party or fully opt-in.

Defenses can be strengthened, but pre-placed units can't move too far from their original positions.

Since the winrate would be skewed toward commander players, it would be important to carefully frame goals for the opposing player and provide a supporting reward structure. The goal isn't to actually "defeat" commanders, but instead to make them work for the win, and prepare nasty surprises on the way.

Killstreaks underline the importance of kills

In my map there is no loss condition for the opposing player, instead it gets a score based on the supply of enemy units killed, with a bonus score for preventing commanders from completing bonus objectives. Only a small bonus is given for actually defeating commanders.

Ending screen for the opposing player. Kills are at the forefront.

There are a lot of interesting design choices when shaping the gameplay for the opposing player – related to economy, objectives, top-bar design, unit and attack wave limitations – too many to go over here in detail.

Protecting the main shard is crucial. Smaller shards expand the economy.

My map is nowhere near perfect, but a mode like this could add more replayability, a bit of challenge, and limited competition while keeping the standard Co-op experience and mission flow the same. It might be worth exploring further.

Both this and the survival mode could be in theory added on top of regular Co-op missions. The advantage of doing this is that maps are reused, and player matchmaking isn't split. If a mode is popular enough, it could get a separate matchmaking queue.

Other ideas

A few more ideas to explore in Co-op. Some might fit in the final game, others might not.

  • Ability to play a certain class of user-created maps with your unlocked and leveled commanders (likely without any experience reward). This could mean having a simple framework for mapmakers to create maps using Co-op dependencies and automatically pull player progression from player's profiles (bought commanders, levels, and customization).
  • The campaign could unlock and level certain commanders later playable in Co-op. This would provide a natural transition from Campaign into Co-op. Although power levels and commander design would either have to fit together, or the difference would have to be explained.
  • Co-op commanders could be available in the variety mode with weekly changing rules.
  • There could be a mode that links several missions together (~beehive or challenge mode). However, there are some challenges to implementing this in multiplayer. I also had a competitive version of beehive that could work for clan wars.
  • While two players might be chosen number for Co-op, it would be good to build foundations agnostic to the number of players. That said, making Co-op and its maps scalable between let's say 2–4 players is a daunting task. It would require dynamic changes to base layouts, enemy difficulty, mission pacing, mutators, and it might even limit commander design. On the other hand, splitting the map pool isn't the best idea either, as it would mean either a lot more work or limited map pools.
  • A queue for player-made mutations. Authors choose a map, mutation name, mutators, and other options. Mutations can't repeat for one player, less popular would get phased out, new and popular ones would be more likely to show up. There would be limitations on the creation itself to limit difficulty. Now and then you could add your own mutation to the pool. If you liked one you have played, give it the thumbs up and the author will get notified.
  • More random elements in maps. Challenging bonus objectives could be a good target for randomization as they won't mess up difficulty too much. Random weather could play some role visually and mechanically.
  • Improved custom mode – option to play solo, select enemy race, composition, and map pattern. There could be some achievements related to this mode (e.g., for beating a specific mutator combination).


Thank you for reading. I believe Co-op has a lot of potential, and while it was a big success in StarCraft II, it can be improved further. I'm looking forward to seeing how future RTS games will tackle it.

For discussion check this thread on r/FrostGiant.

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