There was an idea for something like an academy, which would be in the format of: many short custom missions. They would be like campaign missions, except:
-no multiplayer, single-player only
-they are designed to last only a very short time, like 5-10 minutes, not be like traditional campaign missions that can drag out to 2 hours if you want to turtle up and take it slow. If you want to teach them how to hurry up an air factory the "win condition" / "lose condition" could be something like "shoot down the enemy bomber before the 3 minute mark, and then capture the enemy radar by the 4 minute mark" (which would be impossible to do without using a transport, design it so an engineer or ACU can't walk fast enough.
-they are designed to teach specific skills (specific builds, certain kinds of micro) and it's not meant to be a secret. The FA campaign missions do a little bit of this, teaching people how to use new unit types, but this would be the entire focus of the mission and the player would be told: "this is what we want to teach you"
-no real need for a cohesive plot/story at all
-it should keep track of your progress to say which missions you've beaten, or not. This would help players to get a sense of accomplishment by completing the training missions, and help them to see which one they should do next
-avoid using voice acting or in-mission text. Keep most of the text/explanations outside of the mission, that people can read before they launch the mission. this also makes it a lot easier to support multiple languages. It has to be easier to translate a 4-paragraph block of text that's visible in the client than it is to translate a bunch of in-game text messages to the player. Also, the client itself could link to youtube videos showing the mission being completed successfully by a competent player. So if someone can't beat it on their own they can watch how to do it and then copy
We could even make a "rating" based on how many of these missions a player has successfully completed. Basically this would be done to encourage/shame weak players into doing the academy missions so they can learn all this stuff and we don't have to take the time to teach it to them. It would be an alternative way to build up a rating so people can see your skill level. And perhaps this "academy rating" should be hidden for everyone who gets to at least 500 rating points. There's no point in showing the academy rating for people who have managed to get their global rating up high enough.
It's a way to feel accomplishment if they're feeling bad about getting their butt kicked in competitive games against people (whether it's 1v1s or 8v8s), and it would actually teach them what they need to know to get better.
Assuming the "academy" was completed with about 100 short missions, the time investment would eventually pay off in a big way I think.
Every time someone proposes this idea, people like it, but it never gets done. I don't know how to make custom co-op type missions, or I might consider doing some of it.