Say you start a game that is just yourself versus an inactive AI.
What I would like to be able to do is bring up the pause menu and mid-game be able to change which of the two players are controlled by me and which are AI. For example as P1 I could build a factory and some units, then pause and change P1 to inactive AI and P2 to myself. Then make P2 make a different factory and some units. Then change back to P1 and have P2 AI again (but this time an active AI). That way I could play against AI but having AI play with whatever starting fac/units I want it to have.
Or I could make two AIs play each other, but first I control each to make them start however I want them to, than see how the game turns out.
This would open up a lot more sandboxing and testing opportunities.