This pisses me off.
Portal doesn't need another sequel because, unless they can find an absolutely fantastic way to continue the story following Portal 2's excellent closure, they're going to end up making a really fun game with a half assed (excuse the pun) story line.
Think of it like this:
Back when Portal 2 came out, Jonathan Coulton outright said he'd gladly create a new song should another game in the series be produced.
To this respect, they already have someone to create iconic, awesome music. Then, they still have at least most of the original teams, correct? The games were fun to play with and is just one example of what might be a good title for their Steam Machines...
Of course, if this is at all reality.