"Son, I need to go to meet a old friend." Chell lightly said to her son next to her. "Ok, Mom, why are we in a desert though." Her son, Antonio asked. "Well, she's here." Chell said, opening a door to the test chamber. Then there she was, GlaDOS had millons of robot testers, it was like a portal world. Antonio only 13 noticed something. "That looks like UltrOS, the robot at the center of our city." Chell grabbed a portal gun. "Exactly." She exclaimed. "Chell? It is you, I see you have a son now... Very pleasing. Welcome to testing land. You can go now, I just wanted you gone the second time." GlaDOS said, shutting the door. "Mom, can I try that?" Antonio asked, he grabed it and shot his white shoe then the white bridge they walked on. Chell fell and came out of his shoe. Then UltrOS saw them. "A Portal gun? Well, you can get me, I'm just an improved version of GlaDOS." UltrOS said.
That is how I think it would be, but we can't be sure, but lets just hope for those two things (Half Life Episode 3 or Portal 3) will come out.