The reason Portal and the portals themselves worked so well inside the game was because Portal introduced concepts slowly. A problem with most Portal mods is there is a difficulty spike after introducing a concept. This is why, in Portal: Rubrum, I am striving more on the challenge than the actual puzzles. This doesn't mean the puzzles will be third-rate, it just means there will be more attention to detail when introducing a new concept.
A new element that will be in Portal: Rubrum is dwizzlers. Dwizzlers are a pretty simple concept: anything goes in and out, the dwizzler will toggle.
For example, if the dwizzler is off, it will be blue.
If the dwizzler is on, it will be green.
I'm planning to make this concept glide as smoothly as possible into the end user, by making simple puzzles showing one feature of the dwizzler, and eventually progressing into using multiple.
