Wan founded the White Lotus
Firstly, this is not a new theory I’ve had, It actually occurred to me back in Book 2 and was reinforced when I re-watched Spirits this past January/February. And was further reinforced during Bk3’s “The Stakeout”.
We know that the Statue of Avatar Aang in Yue Bay has Aang standing with one foot on an open lotus blossom to symbolize the White Lotus coming out of hiding and “openly serving the Avatar”. (exact wording from Zaheer, though we knew this before he said it)
We also know that Wan’s statue is (currently) the only other known statue in the statue line up that is standing on an open lotus blossom.
In The Stakeout, Zaheer spoke of Avatar Wan by name and his struggle against Raava and Vaatu - something he learned about before Korra ever did because the belief that Chaos was the natural order and Vaatu needed to be released/the avatar destroyed was a core tenet of the Order of the Red Lotus, who broke off from the White Lotus before Korra was even born. Which means the White Lotus knew about Avatar Wan and that he split Raava and Vaatu and have had this information for as long as they’ve been around.
Seeing as how Bryke has had a habit of including little clues and hints about things in the series in the architecture and background designs (the lion turtles in Piandao’s home as an obvious example), I don’t believe my theory is particularly far-fetched.