Is Roronoa Zoro based on Rurouni Kenshin?
Although it is never really stated they are &quot;based on each other&quot;, Nobuhiro Watsuki helped in the making of One Piece.

In the manga series &quot;Rurouni Kenshin&quot;, the Straw Hats' flag design was used on a bomb to start a vengeance hit on one of the main characters of the series. Before writing &quot;One Piece&quot;, Oda worked as an assistant to Nobuhiro Watsuki, the creator of &quot;Rurouni Kenshin&quot;.

Also, Nobuhiro Watsuki usually creates his characters based on historical figures, or figures from other anime/manga:

Watsuki bases many of his characters on historical figures, characters from other manga/anime, and video games series. For example, Himura Kenshin was based on Kawakami Gensai, one of the Four Hitokiri of the Bakumatsu.

Watsuki also mentored  Eiichiro Oda, the creator of One Piece. So Oda's creations might have been a bit influenced by Watsuki, making Zoro somewhat resembling Kenshin.

Nobuhiro Watsuki is known for mentoring several manga artists who eventually got serialized.  For a while there was a time when the most popular Jump manga authors were all his assistants.

Eiichiro Oda - creator of One Piece


Sources: Nobuhiro Watsuki, Eiichiro Oda