I would like to avoid a prequel. So rarely does it end well. One developer said something along the lines of "it would be cool to see Garrus in his C-Sec days, see what made him who we love" Hell no. Leave blanks like that blank. Let the fans fill it up if they want to. Trying to explain the magic only takes it away. Look at Star Wars. Had to tell the Darth Vader story, sigh.
Now a sequel is much more interesting IMO. I'd like to see a time skip of say 500 years or something. Follow a person (player can choose race, please) who has lost family to the Shadow Broker and is on a hunt for revenge. The Broker can still easily be Laira. That encounter can ONLY be interesting, no matter you opinion on the asari. The newcomer is confronted with a more standard Paragon/Renegade thing. The old fan comes with baggage I like the thought of that. And I just pulled this out of my ass in 2 min, and I am a horrible writer.
I would much rather that BioWare tries to create new magic, than rely on what already exists. Vote Sequel.