In Mega Man Maker, there is quite a few of ideas that haven't been fully realized, but can still be uncovered hiding within the files of the game. Much of this content can still be accessed in-game with a little bit of elbow grease, but is not available through normal means.
This article is a work in progress. If you know anything about material that exists within the game that isn't accessible, feel free to add them at your own leisure
Caricarry from Mega Man 9 is in the game, though cannot be selected without editing the favorites/recent tabs, and is fully functional; Shooting it will slowly causes the rocks it carries to get dislodged and potentially damage you.
Both variants of the Petit Devils, also from Mega Man 9, can be selected via editing the favorites/recent tabs, and both versions work as you think they would, splitting into smaller devils upon getting hit too many times.
Gemini Man, coming from Mega Man 3, was originally going to be available from the launch of Mega Man Maker, with his data ported from the Make a Good Mega Man Level 2 engine. However, implementing him proved to be too challenging for several technical reasons, and thus was scrapped. Unlike the rest of the unused enemies, attempting to force Gemini Man into a level will cause the client to crash upon loading it.
While not technically an enemy in this game, the custom cursor Pointkun has secrets of it's own. These secrets come in the form of these 14(!) unused frames of animation that could've been used for multiple scenarios, including idle animations or editing text boxes.
- Most .NSF files featuring music from official NES Mega Man games, contains the NES game's entire soundtrack within the same file, causing a large number of official music tracks between Mega Man 1-6 to be accessible, but remain unused.
- "Editor.NSF" contains 12 songs seemingly intended to play in the level editor. So far, only track 1 is used, and tracks 2 to 12 currently remain unused.
- Some .NSF files, such as "Timeman.NSF", "Menu.NSF", and "Tutorial.NSF" contain 24 songs featuring original music and remixes for Mega Man Maker's soundtrack. Tracks 4-16, and 18-24 currently remain unused.
Wily 2 BG Alt
This version of the Wily 2 Background can be selected via changing the values of your favorites/recent tabs, just like the enemies described above. It uses shades of purple that more closely resemble the source game, but why this isn't able to be used is beyond me.
Japanese Easter Egg logo
Though this easter egg was removed when the logo changed from Mega Maker to Mega Man Maker, the sprites are still in the files of the game.