A good word for a pivotal person
Solution 1:
Visionary may serve. From en.wiktionary, noun visionary means “someone who has positive ideas about the future” (but also has senses “an impractical dreamer” and “someone who has visions; a seer” as well). One of its adjectival senses is more-directly relevant: “having vision or foresight”.
I was planning to mention prophet (“Someone who predicts the future; a soothsayer”), but now that's been done.
In some contexts, mover and shaker (“Someone who has power and influence in a field or activity; one accomplishes tasks, who gets things done”) may apply.
In other contexts, kingpin may be relevant. From en.wiktionary, kingpin has a sense “The most important person in an undertaking or organization”. However, note that Wikisaurus happens to list kingpin among the many synonyms of big cheese.
Also consider the following words, whose indicated senses relate to turning things upside down, shaking things up, or leading the way:
• pioneer, “2. A person or other entity who is first or among the earliest in any field of inquiry, enterprise, or progress.” — en.wiktionary
• groundbreaker, “1. First person to do, make or fight for something. A pioneer; a seminal person.” — en.wiktionary
• bellwether, “1. The leading sheep of a flock, having a bell hung round its neck. 2. Anything that indicates future trends.” — en.wiktionary
• insurgent, “1. one of several people who take up arms against ... authority” — en.wiktionary
• subversive, “1. A radical supporter of political or social revolution.” — en.wiktionary
• revolutionist, “1. A person who revolts.” — en.wiktionary, or revolutionary, “A revolutionist; a person who revolts.”
Solution 2:
Some suggestions:
a Messiah - an anointed savior.
The one - while often used (e.g. Google's def) to mean a person regarded as one's destined life partner, it can also mean the one who is pivotal
Game changer. MW defines this as * a newly introduced element or factor that changes an existing situation or activity in a significant way*
Guru - MW: a religious teacher and spiritual guide in Hinduism OR a teacher or guide that you trust
Also comes to mind, fulcrum, keystone, pivot, genius, Socrates (as in another Socrates -- could also be used for other philosophers), sage.