Without giving too much away, the first season is based on his stay with Kaoru Kamiya and how he keeps his vow of not to kill.
He did say that the series' popularity fell to its lowest point during this storyline, as of Rurouni Kenshin volume two, but repeated that leaving it out was not an option.
Given that the manga was still in development when the anime began, the producers started preparing filler material early on.
With his sakabatō broken, Kenshin asks Seikū to make a new sword, but Seikū refuses because he wants to live a peaceful life in the Meiji era and does not want to make anymore weapons.