Would be nice to have a tool with which you could create these bricks in the way that first you place a ghost brick of a 1x1 plate somewhere, move it to the proper location if necessary, and when you press the "place brick" key, you'll place the brick, BUT now you'll have to move ANOTHER 1x1 plate ghost brick around, and this determines the corner opposite to the one you just placed. When the other corner is placed, the portal brick will pretty much just sit there and do nothing interesting, until you use the wrench give it an ID and input another ID to which the brick will connect to, not to forget which surface to use as the portal (N, S, E, W, U or D).
When two portal bricks are placed and they're set to connect (could be restricted so that the bricks must be the same size to connect, too), they'd show a picture of the "other side of the portal" on their portal surface. Upon touching the portal surface, the player would instantly be teleported to the other portal brick, preferably seamlessly (like in some maps that can be made in Hammer and other map editors), and perhaps with a slight nudge so that the player won't touch the portal surface immediately after reappearing (which would make him teleport back to where he started and continue teleporting ad infinitum).
This is the idea I came up with when trying to map out Hitm4n's ubermaze, because some of the walkways and corridors seemed to take up the exact same location in the maze WITHOUT intersecting, and this confused the heck out of me.
This would be quite neat for making the illusion of a building that is larger on the inside than the outside. :P