A story about characters who transmigrate, with every effort made to preserve the original characters' personalities and settings.
World: Final Fantasy VII: Crisis Core
Chess game: Code Geass. This story brings in Suzaku Kururugi's younger sister to open this scenario. Since a chess game needs the two key pieces of a king and a queen, it wouldn't do for only Lelouch to have a younger sister as his queen while my Suzaku Kururugi is left alone as a traitor to his country. So we need to bring in one more person to keep the game in order.