Master Yi and Yasuo (a fan theory based on the two being brothers)
Over the years of playing League of Legends I started developing a bit of a fan theory...
In Master Yi's lore you can see how he yearned to be the teacher of the ancient art, Wuju. Yi had become one of the top teachers in his village and was one of the very few to master the ancient technique. (time skip to the Noxian invasion) Yi's village had been attacked while he was away and the only few survivors had been corrupted by poison.
(Now for Yasuo)
Yasuo was a skilled swordsman who trained at a school in a village that practiced an ancient technique (sound familiar?), Yasuo had mastered the wind and was a very agile warrior (sounds a lot like a form of WUJU)... While he was training Noxious had invaded his village and his school (Noxions, I hate those guys...). He was tasked with guarding an elder of the school, but he had left his post to join the front line (Now its time for my fan theory, Yasuo didn't leave just wanting to fight, but also to go look for Yi who at that time was AWAY FROM THE VILLAGE DURING THE INVASION- Yi's Lore).
Fan Theory including Le'Blanc, Riven, or Janna (surprise guest) Le'Blanc- could have easily been in this portion of the war and used her illusion skill to take the form of the top student- Yasuo. Riven- could have been the one to chop down the leader using her blade skill while being a part of the Crimson Elite (lol skin reference) Janna- Could have been in the village while the poisonous cloud was released. While being in the poison, Janna became corrupted (Yi's lore again). While corrupted, Janna wouldn't have had control over her actions and could have killed the Elder with her wind technique.
Now for the fun part, Yasuo's lore... Yas had been charged for leaving his post and the murder due to his mastery of the wind. Yasuo left the village in search of the true murderer. At this time... Yi returned to the village only to find a few survivors (the council that had charged Yasuo of treason). Yi was in shock due to his student being accused so he left to train out of anger to become stronger out of vengeance of him being accused (BATMAN THEORY). During his training he met Wukong who reminded him a lot of his previous pupil who quickly learned the Wuju Style. On his return to the village, Yi had been given the duty of tracking down his pupil (his younger brother).
Once the two found each other they ended up in the duel and Yasuo defeated his brother, Yone. (Yone - Y1- Yi, the name should make sense now). Yasuo buried his dead brother and left on his journey to find the real murderer... Now for the Project Yi theory. A scientist had read up on the #1 swordsman of Ionia and searched everywhere for him, but alas, all he could find was the grave of his older brother (Yi). The scientist then conducted experiments and brought Yi back to life as a mechanical super soldier (kind of like Metal Gear Rising). In the video you can see Yi fighting a figure, viewed as a rival samurai. In my theory, this is Yi viewing Yasuo as being corrupt while the two were fighting and has a bit of PTSD dealing with the fact that he had to duel his brother.