I have played several times and there is a point of no return. This is when you assault the cerebus base. Before you do that get the multi-player readiness up to 100% and you can also boost your readiness via the omega DLC. The omega DLC confers also an additional 2 powers avail as an option in med bay - lash and flare. These two powers were those that aria has and upon successful retake of omega, become avail. Before that, the best optional power was something that allowed you to basically gain control of either organics or synthetics as an ally for a short duration. It was the most effective tool in dealing with all opfor, including turrets who are very useful in turning on their engineers who set them up. After you finish the game, watch all the credits, there is another scene. Additionally, after finishing a game, you will be reset to the helm of the ship just before the point of no return, before the assault on the illusive man's base.
Warning, I think that this is a bug, the initial reputation of Sheppard is maintained and even though I ended the game and played a new one to get my reputation 100% 'bad' I could not get to the reputation that allows the final option in conversation that gets the illusive man to shoot himself. I had played an entire game on insanity and kept the negative conversation replies, but it was still not enough to enable the final choices. I could see them, but not act on them.