In regular portals, the rendering angle dynamically changes depending on the observer's position relative to the portal, creating an illusion of continuous space. I'd like to suggest adding the ability to "fix" the viewing angle of a given portal - likely through its NBT tags - making the rendered texture look the same from different angles, so that it would appear as a picture rather than actual space.
This would allow for emulating TV & CCTV screens, "magic" moving paintings, and the like.
I imagine such "non-portal" portal would also be cheaper to render (since there's no constant need to recalculate the occlusions and so forth), though that's just a guess.

This mod already has GUI portal functionality, another mod could use IP to render the scene into a framebuffer and render the texture normally to achieve this