In Mega Man Maker, as with any game, there are quite a few ideas that have not been fully realized and assets made inaccessible through further updates. Many of these can still be uncovered hiding within the files of the game, and some can even be accessed in-game with a little bit of elbow grease, though it is not available through normal means.
Currently Unused Assets
These assets exist in the current build of Mega Man Maker but are unused.
The NSF files for the first Mega Man game through Mega Man 6 contain most, if not all, songs from their respective games, many of which are not used in Mega Man Maker itself. This includes stage select themes, password menu themes, weapon get themes, cutscene themes, credits themes, boss select jingles, boss victory jingles, game over jingles, and possibly more.
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.
Attempting to play a level with the unused Gemini Man boss placed, will usually cause the client to crash upon attempting to spawn Gemini Man. By using text editing to place more than one boss, and spawning another boss before spawning Gemini Man, it is possible to load Gemini Man without the client crashing, but as the boss has no data assigned to it, it spawns as a 1x1 invisible solid tile.
As Gemini Man was never officially released, its Level Builder icon goes unused.
There are additional sprites for Pointan with extra functions that include a writing bar made by former beta tester Dimpsy.
Since WreckingPrograms never intended to put these in the game and they were sprited without communication with him, the implementation of these animations was never attempted. Despite this, a few of these sprites can be often be seen used in release trailers for Mega Man Maker.
Japanese "Rockman Maker" Easter Egg Logo
Though this easter egg was removed when the logo changed from Mega Maker to Mega Man Maker, the sprite is still in the files of the game. This easter egg logo was used from versions 1.0.0 to 1.0.9, and has been unused ever since version 1.1.0.
Cirno special logo
In the files, there is a special version of the "Mega Maker" logo with Cirno from Touhou. This image is still in the game's files, but it was presumably left unused due to the game's rename before 1.1.0.
Mega Man 3 Wily 2 BG Alt
A version of Mega Man 3 Wily 2 BG that uses brighter colors, and has a slightly different wiring pattern, but it is unknown why it was not used.
"Prototype Block Dropper" Mega Man Sprites
Among the sprites for the playable character Mega Man, there are graphics for Mega Man attacking with an abnormally enlarged arm. It is assumed these are early prototype sprites for the Block Dropper weapon that were likely eventually abandoned in place of sprites more accurate to the source material.
Junk BG 1 Alt
A version of Junk BG 1 without animation, left static in its "power off" state. Likely replaced with the animated version of the BG later in development.
Junk BG 2 Alt
A version of Junk BG 2 without animation, left static in its "power off" state. Likely replaced with the animated version of the BG later in development.
Prototype Health and Energy Spawners
Unused sprites of three 16x16 blocks made by SnoruntPyro. According to Luigi, these were ideas for the health and weapon energy spawners that were experimented with and not removed afterwards. As of version 1.7.5, these can still be found in the game's files, despite Luigi stating that he has no further plans for the objects and would "rather keep his doodles to himself".
Air Tile 2
An unused alternate version of the Air Tile with the goblin face tiles found in Air Man's stage.
Bright Tile 2
While a tile from Bright Man's stage was implemented during the development of 1.7.0, the team decided to scrap it later due to being potentially seizure-inducing.
Time BG 2
A background from Time Man's stage drawn by beta tester BigMaster. According to him, the background was scrapped due to difficulties in making the wood beams in the background not resemble terrain and in adapting the background to the NES palette.
Clown BG (Alt)
A recolor of Clown BG based on the second part of Clown Man's stage. It was unused as the changes would not be perceived as different.
Wily Machine 4 Starting Phase Icons
These icons were for Wily Machine 4, suggesting that you would've been able to change its starting phase like with Wily Machine 6. As you cannot change its starting phase, these icons go unused.
Previously Unused Assets
These assets where unused in previous builds for various reasons, but are currently used in the most recent build of Mega Man Maker.
Caricarry (Before 1.5.0)
The enemy Caricarry from Mega Man 9, was present in Mega Man Maker in 1.0.0, but would not see an official public release until 1.5.0.
Although the enemy was functional for the most part in older builds prior to 1.5.0, it had some collision and clipping issues when it came to solid tiles and objects, implying the enemy was not finished at this point in development.
Petit Devil (Before 1.5.0)
Both variants of the enemy Petit Devil from Mega Man 9 were present in Mega Man Maker in 1.0.0, but would not see an official public release until 1.5.0.
Prior to 1.5.0, Petit Devil's yellow and green variants were separate assets in the Level Builder. Petit Devil's green asset was edited to contain both the green and yellow enemy variants in 1.5.0, and the Petit Devil yellow asset was no longer needed.
According to Goldstorm, this was the screen shown for users that have been banned online. It is an edited version of a newspaper showing Wily's arrest seen during the ending of Mega Man 6. There are colored squares and numbers around each paragraph and text elements that are placeholders for a banned user's username, date until ban is lifted, and reason for ban. In version 1.7.4, this was used for banned users.
Unused assets from development
These are assets that were previously used and replaced in Mega Man Maker's development.
Thunder Beam Icon (Pre 1.1.0)
Until version 1.0.9, Thunder Beam had a different icon consisting of electricity compared to later versions comprising a single bolt of lightning.
Block Dropper Icon (Pre 1.4.0)
On the same topic, Block Dropper (known as Brick Drop at the time) also had a different icon comprising a three-dimensional turquoise brick block. This was changed to a flat forward-facing brick block with the release of 1.4.0 to make the icon more accurate to its source material, along with also renaming said weapon to match its official name, as Mega Man 11 had not been released beforehand.
Spring Man's sprites
Spriter Schmidkalkan made sprites for Spring Man, but those were not used in the game. Instead, the sprites used are those by Gaem and Sodacoma.
Heat Tile (Pre 1.5.0)
A version of Heat Tile that was used in the game prior to 1.5.0, where it was changed so the upper and lower parts had the bigger brick parts from the source material.
MM2 Wily 1 Tile 2 Alt (1.5.0)
A version of MM2 Wily 1 Tile 2 Alt that was used in 1.5.0. It was replaced by FakeVirus with a version that changed the inner corner tiles.
Cut Tile (PU) (Pre 1.7.3)
A version of the Cut Tile from Mega Man Powered Up that was used in the game prior to 1.7.3, where it was changed due to criticisms for the lack of proper shading.
Block Tile (Prototype)
An early design version of Block Tile that was likely used in developmental builds, it was replaced by a different design in the official release of version 1.6.0.
Bright BG (Pre 1.6.0)
An early, non-animated counterpart to the Bright background that was used in Mega Man Maker before it was replaced in 1.6.0 with an animated counterpart.
Wave Tile (Pre 1.7.0)
This version of Wave Tile was modified by BigMaster during 1.7.0's development to modify the corner tiles.
Beta Fire Wall Modifier
Meka made a previous version of the Fire Wave Modifier consisting in a 1x16 stick before it was replaced by a 1x1 icon by BigMaster.
These are older versions of assets that have since been replaced with new versions of the same asset with different and/or additional functionality added.
These assets, are not able to be selected or used under normal gameplay conditions in the most recent build of Mega Man Maker. They remain in the game to ensure levels made in older builds do not crash when loaded in the most recent version.
Petit Devil Yellow (Never Officially Released)
Prior to 1.5.0, Petit Devil Yellow and Petit Devil Green where separate assets in the Level Builder. After 1.5.0 released, Petit Devil Green asset was edited to contain both the Green and Yellow enemy variants, and the Petit Devil Yellow asset was no longer needed.
As Petit Devil Yellow as an individual asset was never officially released, it's Level Builder icon goes unused.
Checkpoint (Before v1.4.0)
Prior to 1.4.0, A Checkpoint could only be used once when playing a stage, unless the player lost a life or restarted the level, triggering the level to reload and allow the checkpoint to be usable again.
Boss Doors (Before v1.5.0)
Prior to 1.5.0, Horizontal and Vertical Boss Doors could be entered even while a boss battle was in progress, or while mini-boss enemies where spawned within viewing distance of the doors.
Rotating Platform (Before v1.5.0)
Prior to 1.5.0, Rotating Platform only had a singular color variant, being Red.
Due to conflicting ID values with an asset in a different category also having color variants, the newer Rotating Platform asset needed to be set to a different ID value if it also wanted to use additional color variants.
Teleporters (Before v1.5.0)
Prior to 1.5.0, Teleporters where able to send players to disconnected screens even if a boss battle was currently in progress. Also, the asset only had a single colour variant, being pink.
Wanaan (Before v1.5.0)
Prior to 1.5.0, Wanaan only had a singular color variant, being Dark Green.
Due to conflicting ID values with an asset in a different category also having color variants, the newer Wanaan asset needed to be set to a different ID value if it also wanted to use additional color variants.
Water (Before v1.5.0)
Prior to 1.5.0, Water only had a singular skin variant, being Yamato Water and was listed as a Level Object. However, to adjust Water for different skins in update 1.5.0, a dedicated folder for Water in "Tiles" was created while the Yamato Water tile in the Level Objects folder became inaccessible.
Conveyor Belts (Before v1.6.0)
Before 1.6.0, Conveyor Belts only came with the skin from Metal Man's stage. To implement its change, the previous version of the Conveyor Belt was made unavailable for backwards compatibility and a new version was created.
Further compatibility was added so that legacy Conveyor Belts could change direction if Metal Man's new "reverse Conveyor Belts" ability from 1.6.0 was enabled.
Crabbot (Before v1.6.3)
Due to a design mistake, the enemy Crabbot's initial horizontal movement direction is opposite of its setting in the Level Builder. The original mistake version of the enemy was left in for compatibility with levels made working around the issue.
Mechakkero (Before v1.6.3)
Due to a design mistake, the enemy Mechakkero moves in the air even when not jumping, possibly causing collision issues. The original mistake version of the enemy was left in for compatibility with levels made working around the issue.