Dynamic mod downloading and loading would help reduce and remove some of the barriers to using mods in custom missions.
I propose loading and unloading mods dynamically as needed, as well as downloading them from the steam workshop as a method to reduce the restrictions imposed by having to do a lot of work to install mods to use for individual multiplayer missions. This will help improve diversity in multiplayer, as well as make it easier for players to discover new mods and missions to play.