Good news! There will be more Portal. Bad news! We have no clue what that means or when it will happen. Valve spokesperson Doug Lombardi recently told Eurogamer:
There'll be more Portal, for sure. But the details of that, to be honest, we're still working out.
He adds that Valve wants it to be more than "a bunch of new puzzles." And we must agree, with Portal's story being as compelling as its gameplay—even if a sequel or deeper add-on couldn't duplicate the experience—releasing more levels alone reeks of wasted potential.