System Dynamics remains a woefully underused theoretical system for modeling complex systems and understanding the feedback loops involved and the way the structure of the system influences the outcome. I know of no one that explains it more clearly than Donella Meadows (RIP).
I just finished reading her book, and I am trying to dig into this field to learn more about it, as I consider it pretty interesting. I am still surprised that we don't teach it at school (high school). I want to believe it will happen in the next 20-25 years.
I say this as someone who is prepared to consider the idea of systems thinking as a "second tier" cognitive strategy, as advanced by Wilber etc. I can't imagine that idea getting mainstream traction.