Portal 2 was challenging enough when all you had to worry about was space, but now we have to think about time as well?
Portal Reloaded is a mod, released just in time for Portal 2's 10th birthday, that introduces a new set of test chambers and, more importantly, a new portal colour. You’ll still be using the old blue and orange ones, but the green one you’ll also get will let you move through time, as you set up puzzles in one timeline and then move them along/solve them across two different eras, set 20 years apart.
It’s even got new voice acting and an original soundtrack. You can get it on Steam here.