This post contains a heavy KotOR spoiler so if you still haven't played the game and you want to, you should play it first. But then again, when I first played it it was spoiled for me too and I still enjoyed it so what the hell, do as you wish!
This is how I think Mass Effect happened:
Okay, let's make a game. It'll be a space opera type of thing.
The main character will basically be a hero (male or female; for simplicity, I'll use the male form in the text) that is selected for the mission based on his past. His name will be Revan. What? Sounds too fantasy like? Okay, let's make it more human. He'll be called Shepard.
He will report to a special group of mixed species council of wisest people around, but they will, of course, doubt him a fair amount. We can't make it all too easy.
He will be a commander of the fastest and best ship in the galaxy. We'll call it Ebon Hawk. No, wait. Let's call it Normandy! Of course, the ship won't ACTUALLY be his.
He will bring with him a crew of both human and alien species.
His main enemy will be a former good guy of high rank. Let's call him Malak! Actually Saren sounds better.
The reason he will become evil is the mysterious space ship type of thing that's able to create incredible 'man'-power force in a very short time. Let's call it Star Forge. Hmm.. It doesn't sound flashy enough. Let's call it Sovereign. That's more powerful. One word.
But we must add some more content for sidequests. Let's add a special scientific organization that pretends to be good, but it's actually evil. The name shall be Czerka. Wait, actually that sounds non-english. Let's call it ExoGeni.
We also need to have an evil organization that will make things complicated. They will be called The Exchange. Actually that name sounds confusing. Let's call it Cerberus. That sounds mythical.
Now that sounds awesome for a start. I can't believe how no one thought of anything like it before!