What was the Japanese exclusive content in Cave Story 3D?

According to the Game Releases page on cavestory.org, the Japanese version of Cave Story 3D included extra bonus content.
Cave Story on the Nintendo 3DS (physical release) with new 3D graphics, new or expanded areas, slightly increased difficulty, new item locations, new music remixes and a time attack mode.
Japanese release exclusively contains crossover content from Crazy Climber, Ikki, and Dragon Slayer.
This exclusive content was briefly mentioned in an article by Siliconera.
The Japanese version of Cave Story 3D will have additional content. Nippon Ichi included retro game collaborations with content from Ikki, Dragon Slayer, and Crazy Climber. Right now, Nippon Ichi is keeping the details of these collaborations under wraps.
This content was also mentioned in RedRiderX's Cave Story comparison guide, but once again, no real detail was given.
Numerous easter eggs for old fans of the game and "cross-over content" with nods to Prinny in the western version, and content from Crazy Climber, Ikki, and Dragon Slayer added to the Japanese version
What Japanese exclusive crossover content was included in Cave Story 3D? Was the crossover content significant, or was it limited to minor easter eggs?
Pictures about "What was the Japanese exclusive content in Cave Story 3D?"



What does Cave Story Plus add?
It features remastered graphics and music as well as several new game modes, one of which being exclusive to the Nintendo Switch version.What was Cave Story Made with?
This was a free game created in cooperation with Studio Pixel, and was available for a limited time, starting 9 December 2021.How many levels are in Cave Story?
15 levels to explore in the vast world. 10 unique weapons to find and upgrade. USB controller compatibility. Four unique endings.How many hours is Cave Story?
When focusing on the main objectives, Cave Story is about 7\xbd Hours in length. If you're a gamer that strives to see all aspects of the game, you are likely to spend around 14\xbd Hours to obtain 100% completion.Why Cave Story 3D Sucks
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Ketut Subiyanto, Marta Wave, Keira Burton, Monstera