Why are the league champions fighting? (theory)

Someday in the future, a threat poses to destroy the universe. To combat this, Bard, Ao Shin, the Darkin, the Watchers, and other powerful immortals go back in time to alter history and create "champions" to fight this threat. Bard goes throughout history, searching for champions created by the influence of his allies, and brings them to the celestial plane. He informs the champions of the threat. Begrudgingly, they put aside their differences and agree to fight for the cause. What we see as "League of Legends" is in fact the training the champions are undertaking to prepare for this threat.