Several interesting points in the Aoyama's interview from the book Heiji and Kazuha Secret Archives:
About M21
• Aoyama drew a few original scenes for M21, and also revised a lot of scenes.
• The most recommended scene in M21 is the Karuta game. The teaser poster shows Kazuha of one moment during the game.
About Ooka Momiji and Iori Muga
• Way before M21 was planned, Aoyama had thoughts on introducing a certain "Ooka" as a rival of Kazuha into the plot at an appropriate time, based on the well-known ancient judges Ōoka Echizen and Tōyama Kinshirō from TV series, etc.
• Momiji is from a rich and powerful family. She is richer than Suzuki Jirokichi. Momiji doesn't do any kind of martial art (karate, aikido, etc.).
• Iori was at first designed to be an old man in the script of M21, but then Aoyama changed him to a young cool man. Iori had all question marks as his names in the character name box when he was first introduced in the manga. This kind of trick had never been used before.
• When asked about whether Momiji and Iori will have a romance in future, Aoyama doesn't provide a direct answer, instead he says that Momiji is truly attracted to Heiji, and that Iori is a faithful butler to Momiji. Aoyama has no idea about Momiji's personal life at present.
About Okita Soshi
• There's a chance that Okita may get involved in Heiji and Kazuha's relationship. Okita calls Kazuha "Kazuha-chan", seemingly more intimate than Heiji.
About Heiji and Kazuha
• Heiji somewhat grows up a little in the relationship. Now he's sure about his love feelings for Kazuha.
• The name "Kazuha" comes from the wife of Aoyama's assistant.
• Aoyama's favorite scene of Heiji and Kazuha is the one on the cliff from the Mermaid case. Fans used to ask about how the two were saved, and "it's the guts (or willpower, 根性)", says Aoyama.
• We'll see more and more stories in the romance of Heiji and Kazuha. Momiji will also be involved. It's unknown whether Okita will get involved.