While sketching a profile to be used in 3D, in order to create an empty region that will be easily picked for the 3D operation we often use Trim, however, trim a sketch entity makes it lose its parametric constrains, constrains that would be useful for later modifications to the profile.
My Idea/suggestion is that when an sketch entity is trimmed the entity becomes a construction line retaining the constrains and the untrimmed portion of the entity becomes coincident to the construction line. So, changes to the constrains of the construction line update the untrimmed portion. Obviously, this wold require that the trimming element would also remain as a constrain.
An implementation of this would be a line at an angle, say 30°, that is intercepted by an horizontal line, I want to remove the portion of the angled line that is 'south' of the horizontal line, so I trim it. Say that latter I wan to change that angle to 45°, maintaining all the rest. With my idea this would be something very simple to do, that wouldn't requiring resketching that portion.