A good starting point, if nothing else
Unlike buses, Valve games do not come in threes.
It’s totes on the cards, is what it sounds like.
At one point it had bullet time.

So you know, it’s good we have this idea but there’s a lot left to do.
Someone’s gotta think up some new portal puzzles!
But we do have an actual idea, yes."

It’s not unknown, but there’s no formal pitch process at Valve either.
It’s always kind of a, you know, grassroots campaign I guess."
By the time they’ve done that with a sequel, they’ve already fulfilled their creative vision.
From a writing perspective, at least, Wolpaw and Pinkerton are still keen (as is Glados).
It’s neat to hear about the ideas that got cut from Portal 2, too.
You’ll find plenty more details in DidYouKnowGaming’s vid above, and even more inthis one here.