For a previous rendition of the same concept, Dorkly made a similar video, that I think is much more enjoyable (as Mario himself takes humor out of his having the gun); although , the Dorkly version didn't attempt to make the game play "work" (this version from Stabyourself actually looks like something that, if built, could actually be played; the Dorkly one is "just" machinima).
One additional difficulty in making Portal 2-dimensional is the loss of a relative frame of reference, which can cause control problems. If you hold down the left key to enter a right-facing portal, and exit through another right-facing portal, do you start moving right even though you are holding down the left key? Massive kudos if they can find a intuitive way to solve this.
Since we're having a bit of fun, my suggestion would be to horizontally flip the level entirely. I suspect it would rapidly become more intuitive than you'd think; a shock the first couple of times, but just "how it works" after that.
Further edit: I have no idea why, but in my head the color palette shifts when the screen is reversed, like all the colors get 20% darker. (Perhaps I'm analogizing with A Link to the Past's "Dark World", which isn't really the same thing, but you are welcome to explain that to my subconscious at your leisure.) It doesn't seem like there's a good reason for that from a Portal point of view, but it would offer a subtle hint that you're supposed to be going the other way now.
You wouldn't need to change the color palette. Freeze the gameplay and animate the world "flipping over" as a big 3D surface, like in Paper Mario. Continuity is established, problem solved.
Would you have the controls also be reversed (left becomes right and vice versa)? I'm not sure what makes most sense. Maybe you'd get used to it after a while.
What I am planning to do is make it like in games that have fixed cameras: When the camera changes, the character keeps walking into the same direction unless the player presses in another direction.
No, just the one. You step the portal and the only the world reverses. You keep holding right, and you go right-original, through the portal, then right-mirrored, but it's always right, so there's always agreement between the user's direction and the motion of Mario. It's the world that gets flipped instead of your control scheme.
The idea that Stabyourself puts forth is arguably the default answer, in the sense that it is the usual one, but while basically functional, it never ever feels right. I just got done with Catherine recently which does the same basic thing (albeit with going behind something instead of going through a portal, but it's the same reversal problem) and 18 hours into the game it still felt unnatural... and that's hardly the first time I've encountered the "keep going a certain direction while held, then completely rekajigger the interface at an arbitrary point in the future when the user recenters the controls", either, so it's felt unnatural for even longer than that. (For bonus points, play on an Atari 2600 controller that just likes to cut out for a frame or two every so often, so the effect is that the game's controls appear to simply spontaneously revert.) Flipping the world would at least be changing things up.
portal: the flash version seems to work well enough.
The directions are constant, but the momentum gives you time to shift your heading as you enter/exit portals
I like the idea, but the levels would have to be entirely redesigned if you wanted to preserve good gameplay. Having a portal gun tacked on to Mario, with no need for it in the game and levels not designed around its use, would just kind of suck.
If the game should be played more then once or twice the design must be changed in some points. Other levels, enemys and events might be nessesary to keep the fun high. But if the intention is just a nice gag without the need of much replayability it seems to be totally fine to just simply combine the two.
Fun fact: In the forgettable geography game Mario Is Missing Bowser used a network of pipes called the Portably Operated Remote Transport And Larceny System, or PORTALS, to invade the world's cities and steal their artifacts.
If the diversity of hazards was ramped up this could easily be very fun, a much more intense and fast-paced arcade take on Portal. Never mind the classic sprites and music, the format of Super Mario is just different and would amount to a different Portal.
But as of now, most of the enemies in Mario were designed to be adequate hazards for a character who can only walk and jump, the matchup becomes pathetic if he's suddenly equipped with a Portal gun. From the video it seems the game-makers will have to be more creative with how Mario can be killed (Great heights, ranged attacks, time-trials, as opposed to just touching a mushroom/turtle/pipe plant)
At first I thought it was just plain stupid, to combine these 2 concepts. But after watching the video I can really believe that it will be a lot of fun! Kudos for that idea!
http://www.dorkly.com/video/14583/dorkly-bits-mario-with-a-p...