Missions in this context is a term that could be applied loosely - you dont need to have lots of new content generated, you can just have a handful of maps/AI setups picked for each AI , and those are then ordered based on difficulty and the client has some sort of tracking feature that takes you through each sequentially and (ideally) gives an achievement when you complete them all. The main work for this would be on the client side, but the actual content (AI and maps) already exist.
So for example you could start with a simple 1v1 Easy AI on a 5km map, and work up towards say a 1v4 custom AI on a 20km map. It could be a way of showcasing both custom FAF maps, and the AI at the same time (since PvE players are less likely to have seen the maps that feature in the main matchmaker). It could even be combined with other mods (where they're compatible with the AI), such as the survival mode mod, or mods that add new units.