Luigi's Dream World
I feel we have enough competitive stages as is (I counted, about 22), but I like this layout enough to submit - basically, it's Smashville but the platform is a bit smaller and moves very differently, over a bigger stage. The platform stays in place on either the left or right - further than Pokémon Stadium but nearer than Kalos, and every 10 seconds it shakes to signify that it's about to move very quickly to the other side. The platform is a blue-outlined sleeping Luigi, upside down to better fit the hitbox...
and the hard platform is a bunch of Luigis! These Luigis are holding each other up, but are still sleeping.
When the platform is shaking, the sky briefly turns dark and the Luigis open their eyes in shock as to imply a nightmare.
Snappy name, eh? You fight atop a big sticker book - it's not in the middle page so the platform is uneven. There's a fan in the background that will blow foes away a la Whispy Woods. Red platforms are soft as usual, but the yellow blocks are stickers, which capture foes and leave them vulnerable. If not hit while on a sticker, you'll be launched back onto the stage - unless the fan's in your direction of course.
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The book itself does not have the pop-up grassy-field appearance of Super Happy Tree, it's just a book. The background is taken from this screen, it moves diagonally and occasionally changes direction. There's glitter perpetually dropping from the top.