What about a non-portal themed map + timetravel?
Some player starts to cry when they see maps with style/atmosphere that not present in the original portal game, like angled surfaces, a destroyed room after a clean one, or never seen game mechanics and puzzle solutions. My first P2 map (that was mainly a testmap to try out every method and style before the official SDK) received some of these too, even my 2nd one where I had to follow one specific style (wheatley clean style) because of the contest.
Now I planning to do a halfly outdoor map, like an ancient maya ruins or other destroyed castle-ish map, with a huge hole above a (collapsed) cave where the sun shine in, lots vegetation, a map in a deep valley with lots small "dirt mountain", trees, small puddles, ruin remains, like a very small un'goro in WoW 
So basically maps where you cant climb or fling up or out, but maybe will use a huge skybox or a 3d-skybox so the player can see far up-ahead. So no portal story, maybe a totally different one, including some time travel to the future during the map. NO portal storyline. IF you wonder what puzzles can be in a halfly-open map, u'll see.
But if just a few likes this style, maybe I'll drop the idea and will just make normal chambers with big bombarded ceilings... I really like maps with lots green, for example the first 3 map in portal 2.
BTW here is the map in case u wanna try it. Just a testroom for timetravel where you can run and look around, nothing else
You can run aroun in both room, nothing else.
This would make a great mod.
Be careful. Skyboxes are broken in Portal 2. So are displacements. 
Stuff falls through them. Skyboxes misload and cover everything. 3d ones glitch through portals.
Stuff falls through them. Skyboxes misload and cover everything. 3d ones glitch through portals.The original portal had the same problem too, but it turned out good.
Ok, then without skyboxes. Displacements are ok, since rocks and grass are not portable, and just need to leave at least 96-128 unit distence between them and the portalsurface, or just use invisible-textured blocks near them.
The outdoor map sounds like it could be good but I love your time portal and it is the kind of map I have wanted to see more of since Time Arch back in the summer mapping initiative.
its just a room where I wanted to test how a portal should look ingame. I will use it in my next map after an "escape" part where you break out from a "prison" without a gun (as a robot) and while the facility collapses and explosions/debris everywhere u find a lab (a modified smaller version of this testroom), activate the portal and travel ahead in time, and then without a portal gun at the beginning, solving puzzles (mainly fixing rooms by for example picking up indicator stripes from the floor and "placing" them where they fell off, putting the red button thing on the button base, etc... later with the portalgun, some ancient ruin in caves with collapsed ceiling, maybe one extra timetravel further to the future, where the portal is in a hi-tech base now...
but these are just concepts, and definetely not in the portal universe and with not portal-style maps... and hopefully ppl will not cry why there is a cave-style texture in an outdoor area or some wrecked car etc.
The ideas I wanna include in my #3 map are those I drawed on paper 2 day after portal 2 launch, and some of them are even from my portal1 maping 4 year ago (they still can be done in P2). I did not checked custom maps in the last 3 month, so I dont know if anyone used any of my ideas yet, but I dont really care, I include them anyway
ps: updated the testroom with hdr
I would play this, it would be nice to see something different. theres a lot you could do with this.
I think in terms of boolean variables. Generally, it makes things easier.
I love that 