During the Mass Effect trilogy you fight a few friendly species "re-appropriated" as bad guys by the Reapers. But not all of them. So concept artist Andrew Ryan decided to imagine what it would be like if the Reapers had been a little more thorough.
There are some major and predictable races on show, like the Krogan and Drell, but awesomely some other more unexpected foes, like the Hanar and even the Volus.
You can see more of Andrew's work at his personal site or his DeviantArt gallery.