What if, in the next iteration of Portal, the portals were four dimensional after the fashion of the "timegates" in Stross's Palimpsest story? Mechanics-wise, placing the portals would become a difficulty. Would you be confined to suitable surfaces in eye-shot, and how would you control when the portal opened to?
I just re-read the end of the story, where he's really making you grapple with all the twisted convolutions that that possibility has to offer. I imagine you could make some really bizarre puzzles if you could wrap your mind around it, which I'm having some trouble doing.