Taisen! Bomberman

Three of the new playable characters are crossovers from Konami properties, transformed into Bombermen! Goebon is a riff on Goemon, star of the Ganbare Goemon / Mystical Ninja franchise; his Super Bomb is a Ryo coin, the series' currency and impromptu projectile.
Light Bon and Pastel Bon are the starring pilots of cutesy shoot-em-up Twinbee; the faces of their mechs TwinBee and WinBee adorn their respective Super Bombs. The quote in Pastel's bio is a reference to the series' catchphrase, "The motto is -Bee!" (合言葉はBee!)

As of Version 1.1.7, most of the game's data is stored in its internal "AssetsBundle" directory, containing files which can be decompiled with the likes of DevXUnity-Unpacker.

unused Characters

Star Bomber has all the graphics necessary to be a playable character (new stunned and victory animations, a Super Bomb, and the appropriate mugshots for menus and Chance Battles), and even appeared in promotional art before the game's release... but as far as I can tell, he was never actually added to the character roster!
The 8-bit Bomberman also has the same assets, even though it only existed as a one-off April Fools reskin.

unused stages

It's possible the material below was used in a previous version or during an Event, in which case it's simply not accessible in the current version anymore. I've had a dickens of time researching even simple things like what features the Online Battle had, so it could just be nobody's documented these!

The "stage" bundle contains three tilesets that, as far as I can tell, are not attached to any stage or event. "Stage08" is a regal castle, "Stage09" is a factory, and "Stage11" is an 8-bit tileset in homage to Bomberman on Famicom. "Stage12" is a screenshot of the Conveyor Belt/Warp arena with "Space Stage" labelled in katakana, and "Stage13" is blank.

The "item" bundle also contains this "ItemTxt02" sprite sheet, with pixellated versions of all the game's items. It's no doubt meant to go with the Retro stage theme, and it may have been used during the April Fools 2016 event... but if that was the case, nobody mentioned it. Ho hum.

The names above are found in the "ui" bundle, and appear to be stage and mode names for Online Battle; the names in bold are not part of the stage select as of Version 1.1.7. It seems most gimmick stages were to have an extra variant (High-Speed Conveyor Belt), and more gimmicks from Single Game were to be carried over, like Mystery Item.

Notably, there appear to be game modes among the last nine stages that are not accessible in the last version, or mentioned in update changes or events. Crown is no doubt what the PUMA IGNITE DISC event was based off of. Escape requires players to find a key to open the exit door. Point Panel likely hid Point Panels in soft blocks, and whoever collected the most won. Sprites for these objects still exist alongside the item graphics.