As described. Please allow saving multiplayer games on the host's computer then the host would load the game, joining players could pick the team they want to control just like they do when joining a new game (they could just remember who they were, nothing complex needed and the joining players would have to remember who they played as last time, the game doesn't have to do this to keep it simple) and continue their game with AI players.
You can save and load single player games, multiplayer would be the same, just assign controls of different teams to human players per the join/setup game interface.
I play with people who are new to RTS and gaming and we love playing coop vs the AI. The problem is that it takes a long time to build a good base and go to higher tech so we want to be able to save and load the game and continue later. We like to play with different units and strategies but not destroy the enemy AI and let them build up again and try other things.
Please please please add local save and load support for multiplayer games.