Depending on the kind of level you are making and the intended difficulty the coefficient can require entirely changing a portion of the shape of a level or the challenges that are in that portion. Finally I test again.
This is when I feel the level is ready for outside testing. I'll ask a coworker to test it real quick, mostly looking for fun vs asshole sections. If a section stands out as being too frustrating, or too easy, I tweak those areas.
This is usually what I would submit to approval and mass testing.
Sometimes, I have trouble picturing a new shape that would be interesting, so I revisit things I've done before and I ask, "are there new challenges I can put here that would change how this plays?" But this is very rare, and a last resort option.