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.
Also 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.
Milestone Celebration: This game is released as part of the Nintendo-declared "Year of Luigi", celebrating the 30th anniversary of Luigi's debut in Mario Bros.. In an unintentional example, the game is this for the Mario & Luigi series, as this game was released in 2013, ten years after Superstar Saga was released.
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.