I figure I'll post these suggestions here.
First off, having floor portals would be a really nice addition. They don't need to have slope support, or be able to be connected to wall portals, but both of those would be nice. Imagine running down a hallway, and suddenly, the gravity shifts out from beneath your feet, and you go falling to the bottom of the rest of it. Heck, if you could cause this to scale your character if you end up falling through one that leads to a smaller one, you could probably pull off some... interesting effects.
Second, through far more unlikely to be done, would be having GIF's store the state of the map, before saving demo information along with it at the end of the file, as that sort of thing might be useful in reproducing glitches. Actually, being able to just save the state of a map and export it to a file to be loaded by another person could cover this too.
Lastly, and this one might be far easier to pull off: being able to modify sprites displayed to singular players, so that, if there was a spy character for example, people on their team would see them as normal, while the other team would see them disguised. This can also be used for other effects, such as displaying a different sprite when it is viewed at a steep angle... though, if that were to exist, it would probably just result in more idiots suggesting loops.
Also sprite and patch rotation, since that would be really useful for nights stages, as an example, or even say, an analog clock in a stage somewhere.
Edit: Just gonna add this new bit here, since it would be better than me double posting...
Recently, in the process of making my map, I ran into some uncomfortable issues in my mapping, and came up with two linedef effects that would allow for fixing these issues, mainly involving curved surfaces. Ahem.
*A Linedef effect to stretch the flat of a floor and/or ceiling in a tagged sector by the distance between its vertices, making the flat repeat after reaching that point. a distance of 0 on either the X or Y axis resulting in a normal stretch on that axis.
*A linedef effect to stretch the horizontal and/or vertical length of a texture on a tagged linedef, using a similar method as the previous effect.
These two effects would prove invaluable, as in case of the first one, this can be used to align the bricks of a flat with that of its neighboring wall, which is especially useful in regards to slopes, which when using them in the shape of a curve, heavily deforms the texture when the slope gets to its steepest point. Where as the second effect is useful in the situation of aligning textures, incase you have a slanted brick wall, and want to keep alignment with the bricks on the floor, either with the flat walls on either side of it, or the slanted wall itself. (such as with 45 degree angles.)
I have no idea if you could adjust the scale of connected linedefs if they were all tagged the same way, but if that could be implemented, it would make things a great deal easier, especially in regards to curved walls.