In my opinion there shouldn't be separation for solo or coop runs. By default it should  let a player take the AI spot if you want to be online.

 Theres no reason you should have to wait for players to join before being able to start a run.

If i have a multiplayer run and exit to the menu then reload it, it replaces the player with AI. So why can't this be done the other way around going from solo save to online if i want someone to join my run 

It Would improve matchmaking so much