Pen tool with measurement
When using pen tool the user can type in a specific measurement in which they would like their line to be instead of having to rely on trying to make it exact using the numbers.
Select pen tool -> Click mouse where you want the pen tool to start -> Type and enter specific measurement wanted -> Pen tool will then create the length -> Click and place in desired location.
I think this would be really beneficial when using the pen tool so you are extremely accurate as being a few millimeters off can cause problems.
Well, clicking with the Pen in Illustrator starts the path. The CAD-like approach you offer is alien to the existing workflow and would clash with a lot of workflows. Even as a separate mode, with a modifier key held, it can be hard to do — all of them are taken, except Shift.