According to Oji Hiroi in Vol. 22 of The Super Famicom magazine, Tengai Makyo originally wasn't going to be a series, and was planned to be a single game. He elaborated, saying:

"The first map I drew for the game was way too huge, though, and we split it into three parts. That left a whole section of the game for China, as well as Kyuushuu and the Ryukyu islands. I realized if we left it at just one game, the player would never learn the origins of the Fire Clan or the Book of WareWare."
Contributed by ProtoSnake