I was thinking about the lighting issue, and while I'm a total noob when it comes to OpenGL/GPU stuff but I was wondering if it would be possible to implement something like a custom raycasting algorithm in order to transport the individual "rays" through the portal, therefore giving inter-dimensional lighting.

I'm not sure if this is possible, but it would be so cool be in a room with a portal and see the hellish glow of the nether, or see a beautiful blue ocean shining through lighting the room up like an aquarium.

It's hard to implement and would cost a lot of performance.