chell escapes at the end of portal 2(well, more kicked out by glados), why would she return when she's tried so many times to escape only to be dragged back in? it makes no sense, and glados wouldn't go and double-cross chell and drag her back in, she says that chell wins and that she gives up trying to contain her. also making a new character is out of the question, orignally in portal 2 you played as a lady called mel, but she was scrapped as game testers complained that glados didn't recognise them. whilst they COULD use atlas and p-body, what would the story be? if one of them supposeddly disapeared they could easily be reassembled. and if they made portal 3 people would then beg even more for half life 3 and that wold be annoying. and if you want a prequel, portal prelude is for you. and the idea of the outside being fake is just plain dumb so don't suggest it. also valve could decide to confirm the death theroy that uses greek mythology as evidence, something portal 2 uses a lot, view this http://steamcommunity.com/app/620/discussions/0/846965882693231189/
.oh and btw, the prima dona turret is based on the expression"it's not over, until the fat lady sings" the prima dona turret being instead of "the fat lady", and she sings, so portal is over.
EDIT: i don't know why i put oracle turret you found a secret, you deserve yourself a potato