Probably my favorite thing about Mass Effect as a series is how much attention was placed in the overall background story for the galaxy. It would be awesome to be able to see this utilized in the form of a prequel, but I see one glaring issue that really stands in the way of this happening. For many, the attraction of Mass Effect is the feeling that the decisions you make actually make a difference and have weight. When you do a prequel, how can you make it so the player feels like their affecting the galaxy, but also not running into conflicts with the overall story? I haven't read much of the material on the Illusive Man that is set before he was the Illusive Man, but possibly it could be interesting to play as him when he was Jack Harper? I don't really think a prequel could be a Mass Effect 4 as it wouldn't be a continuation of the story, but it could be a standalone game. Personally, I believe there should be a significant amount of time before BioWare decides to take the step to make a true Mass Effect 4 that advances the story forward.