When I reviewed Sunset Overdrive, the game had an excellent open world that was begging for two player campaign co-op. Instead, it had one of the blandest eight player horde modes ever created. Too many games just tack on online cooperative multiplayer without any consideration of what the mode needs. This weird misunderstanding of what we want in co-op is increasing in frequency, as more and more cooperative games are made.
First off, we want to play together with like-minded players. This really is what developers should consider first when going forward. Halo: Reach had one of the best matchmaking filters by asking you several general but important questions about how you liked to play Halo This helped like minded gamers to team up easily.
This should be a default feature in co-op, especially when the co-op is in the main story campaign. If someone is just there for the action, then pair them up with other people there for action. If someone cares about the story, get equally considerate players on board with them.
We also need goals worth playing for. The point of cooperative multiplayer is that you are working together, towards some end. This is why co-op in campaigns works so well, and why standalone co-op modes that are barely connected with the main game fall apart. Some games like Halo 5: Guardians have been making strides to close the gap and integrate co-op into their stories, but we still have a lot further to go.
Still, making players work for a narrative goal might get them through once or twice, but we need consistent, enjoyable reasons to bring friends along. We need new tactical options to open up in cooperative shooters. We need new dialogue choices in cooperative RPGs. We need incentive to play in co-op that offers a different experience, without cutting players out of every option. The benefits should be realistic to the player count.
Online co-op has been evolving at a fast rate, ever since Halo 3 and Borderlands popularized it. Hopefully that means these growing pains can be passed through just as quickly.