Everybody knows ITFs origin. The question is which substantial improvements Choi brought to the table,.
Well, my response initialy is how anyone would: A. Quantify what is an "Improvement" let alone a substantial improvement; and B. What would be the answer for the same question asked about Kano, Funakoshi, Ueshiba, or the KKW.
With regard to General Choi, things he did:
1. Changged from only relying on being "Deeply Rooted" so popular with Shotokan, to having a more mobile system, even while airborne;
2. Unify and codify a system from Fragments as practiced in Korea so it could be taught uniformly on a wide scale, setting forth technical parameters for attacking tools, how to use them against which vital spots, stances and purpose for the stance.
3. Develop instructors and dispatch them throughout the world to demonstrate and teach
4. Recruit instructors practicing the root systems and have them come on board to spread the art on a unified scale.
With regard to #2 above, having trained with old time Korean isntructors the lessons were "Like this" or "Not like this" . Seldom was there an explanation. General Choi gave reasons for his methodologies, which was unusual, which is not to say that reasoneable minds would not differ about whether they agrred with him, but at least he gave reasons. He also welcomed "Good" questions whereas in other schools asking questions was often considered disrepsectful. He also employed the socratic method when teaching.
Examples: Low outer forearm Block. Block stops with thumb knucle even with center line. Other systems had it stop over the front leg. His reason was the abdomen is exposed if the arm moves further out. Leaving it there continues the protection.
Walking Stance. Lead loower leg angles rearword so kneecap is over heel. Other systems, lower leg is vertical. His makes it much easier for lead leg kiscking withou as large a weight shift.
Since his extensive text contains 15 volumes the above is certainly not meant to be exhaustive.