Lore Theory: Janna Is A Shuriman Deity
As we've learned from the new Ezreal update, Zaun is actually built upon the ruins of an ancient Shuriman city.
From Janna's lore update last year, we learned that her name is Shuriman for "guardian" and before settling in Zaun, she helped sailors and explorers adventure across Runeterra. While her lore doesn't state exactly when her "birth" happened, we do know that she was active well before the fall of Zaun, and before Piltover was built on top of it. From her lore it, states that much of her worship came from sailors who looking for a quick voyage, since before Piltover existed, the isthmus the city is now settled on made passage impossible, forcing sailors to sail all the way around the Shuriman continent. So now we know that aside from having a Shuriman name, and being active well before the modern day Zaun which we also know is built atop a Shuriman city, that she worked in close proximity to Shurima during her early years.
In the real world, the easiest and most effective way of spreading a culture's faith was by trade and travel, for example, the Greek goddess Aphrodite, who has roots from the ancient middle Mesopotamian goddess Inanna, or sometimes referred to as Ishtar. Janna, being a goddess that was heavily worshipped by traders and explorers, would have a wealth of opportunities for faith in her to spread, which no doubt may have happened. However, we do have to consider the Shuriman faith.
While there has yet to be a canonical perfectly mapped out timeline, we do know that the ancient Shuiman Empire, in place of more traditional gods, worshipped the Ascended, mortals gifted with god-like power and near immortality. How long age the Empire, and for how long exactly, the Empire was active is unclear, but we do know that when it fell, the Ascended went from being worshipped to almost outright hated in the centuries to follow, with few alive to tis day, though this is a story for a different day. The point is that when the Ascended fell from glory, and the Empire fell with it, it would leave the Shuriman people without any organized or practiced faith. It is around this time that I believe Janna may have first sprung into existence.
This is the basic rundown of my theory:
After, probably a century, the fall of Shurima, the people of of Oshra Va'Zaun, the ancient Shuriman city predating present day Zaun, with no more Ascended to worship, look for a new faith. From their search of faith and prayers Janna is born, and as the the city, more specifically its sailors, become more dedicated in their worship of her, the spread of her faith begins. After her faith is spread across post-Empire Shurima, she then recieves the name Janna, and the rest is history.
While Janna is unquestionably one of the longer lived champions in the League universe, she was (probably) not active, or even existent, during the time of the Shuriman Empire, as belief and worship of her would have most likely have been viewed as an affront, and an act of rebellion, against the Ascended. It's after their fall that Janna is born. While she isn't from the Shuriman Empire, she, no doubt, has Shuriman roots.