Given the timeframe, they may not be as recognized as we are able to as this is set in the future. In this instance, Jack would be a character initially released roughly 65 years before this RP takes place, as Mass Effect 2 came out in 2010 and this is set in 2075. I'd assume in that jump into the future, tons of new games would be more appealing to use their avatars, so by choosing stuff we recognize now, it may be more unique as they're older game characters.
But the way I saw it, is that there could be plenty of people with that avatar, but it's like Gmod you know? Even though you could be playing with a distinguished character model, it's still you in the end. I assume the avatar is just the appearance, and even if you used an avatar of someone like Jack, you wouldn't have, say, her powers.