Development Hell: The unique 2D sprites that greatly resemble 3D models in a 3D world is what caused this game to have a long development cycle (that started right after the third game was released four years ago, no less). It got even longer when trying to avert a serious case of Ambidextrous Sprite for the bros.
Giant Giant Bowser, of course, for the result of an already-giant Bowser getting a second size boost at the end of his battle.
Alternatively, Tera Bowser, to reference the final event in Super Smash Bros. Melee where you fought Giga Bowser, Ganondorf, and Mewtwo, and Giga Bowser could possibly grab a random Super Mushroom to increase his size even further.
What Could Have Been: The giant battles were originally supposed to feature the same Sprite/Polygon Mix as the rest of the game. But the team found that this wouldn't mesh too well with the new mechanics they wanted to add, requiring them to hand them over to a separate company to finish up.
Why Fandom Can't Have Nice Things: Because of a bug in the original version of the game, people could post any saved image from the Nintendo 3DS Camera or the Internet Browser to the Mario & Luigi: Dream Team community. Naturally ,everyone started abusing it, leading to the ability to post screenshots in the game's Miiverse community to be revoked on September 15, 2014. The ability was eventually restored, but only after a mandatory update patching the exploit.