Sure! A notable example is Ṣalāḥ ad-Dīn Yūsuf ibn Ayyūb, or, as you might know him, Saladin.

Saladin was obviously a person of great importance in the middle ages, but was essentially forgotten in the intervening period, particularly in the Middle East. He was a ruler from a deposed dynasty, and a Kurd to boot. It is only in the late 19th or early 20th century that he became an important figure because of Arab or Muslim nationalist movements. He was even frequently used as a propaganda piece by Saddam Hussein, which is pretty ironic considering his more usual approach to Kurds involved chemical warfare.

I'm not sure if this is exactly what you're after, but within the field of Bible history, the two discoveries of the Dead Sea Scrolls in 1945 and the Nag Hammadi Library in 1947 were pretty revolutionary - suddenly, these "lost" scriptures were found, and we could learn quite a bit about what these early communities believed, rather than relying (largely) on a crumb of Greek-translated parchment here or a history written by a harsh critic there.

I think the communities at Qumran and Nag Hammadi were important - and we're still figuring out what their existence and apparent beliefs might mean to the theologies of various parts of the Bible - though I'm not sure they'd be exactly "Great Big Deals" at the time. Great small deals?