If you were to teach just one princple, concept, or strategy to another karate student on how to improve their freefighting what would it be?

Angles of defense and attack.

Assuming a moderate level of skill:
Fluidity
If you are tense or stiff, you can not move in a fluid manner, you also greatly reduce your potential for speed, directional changes and reactions from your opponents positions.

I'm also very much in support of Brent's assertion: Angles!

Change-body (or Tai Sabake). Greatly overlooked in many modern Japanese styles.

Combinations, and the ability to use
them adaptively to the situation.

Learning to read your opponent and to be open and receptive to the subtle "telegraphing" that the body gives you of an impending movement.

Timing. In free fighting timing is very important. It is to your advantage to surprise or to get your opponent off guard.
So it would be good to attack an opponent just before, during or immediately after they strike.

Well one strategy or concept would be knowing knowing the best time to attack their opponent. I believe that there are four times that are best for your attack to take place, anticipation of an attack from your opponent, the beginning of their attack, during their attack and immediately after their attack. During these times your opponent is off balance and has already committed himself to movement...More later...Take care...Ken Bright

To have an indomitable spirit.

Kind regards,
Andy

Understanding the ranges of yourself and that of the opponent. Tough stuff, there are so many considerations one needs to be come successful in this arena.