The creators of Sins are to be commended for having such a flexible game-starting set of options. I thought a thread that discusses even more possible options might be enjoyable, although it's likely that most would be of benefit more for customizing Player Vs. AI games. So here goes one idea, and please post more if you have 'em.
One thing I'd think would be interesting to see would be to give the AI more of a head-start than the player, but in a way that doesn't imbalance the later part of the game by, say, giving them an insurmountable economy advantage. For example, have them already entrenched in a component of the system by giving them an already-built small fleet that they can use to attack the player with earlier. Or give their homeworld several orbital defense guns and a hangar that's pre-built.
This could be provided on the start-up screen by specifying a "start-up fleetsize" option for each AI player, with options "normal" (AI starts with same structures/fleet as player), "hard" (AI starts with either defensive structures OR a small offensive fleet, and which one is chosen based on whether the AI is a turtler or assault player), "very hard" (AI starts with both), or "random" (one of the above three).
Knowing the AI *might* attack VERY early in the game could require changes to the perception in single-player that it's always safe to concentrate on expansion for the first ten minutes. It's a whole different ballgame if the first thing the player has to do is a little fortification.