They could always just do what they originally planned to do with Portal 2 and make a prequel that occurs in the earlier days of Aperture Science, likely using that new mechanic they were wanting to use.
Sure, it wouldn’t have Chell, or even GlaDos, but I’m pretty sure we all know that the real star of the Portal series is Aperture Science itself.
(Also, more Cave Johnson, because we all need more Cave Johnson)
With utmost discontent.
Maybe we could play as Rattman in a Portal prequel?
I think the only place left to go would be a prequel. I think Portal 2 ended just fine, although I REALLY would like another one. I still want to know what F-STOP was, because iIrc Gaben said the F-STOP concept was worth the whole creative experiment Valve did. Maybe just make a prequel that is from soon after Bring Your Daughter to work day, and you’re an employee trying to escape. After that, make F-STOP and maybe make that a new series in its own right.
If Valve can continue to do new and interesting things with the portal mechanics, I think there’s still a lot that can be done even as a sequel. I mean it’s hundreds of years in the future, and while post apocalyptic settings are getting kinda stale, I think it would be pretty interesting to explore derelict cities with the portal gun. It could be way more free and open than the last 2 games. It doesn’t even have to be with Chell, it could be another few centuries down the line, and by that time Chell became a mythical figure with a magical talisman, and someone who grows up hearing the myths finds it and wields it in a new story. I dunno what the story would be but I think the concept would be cool to play.