Seriously, why haven’t we seen a world like this? I remember a world on the first Mario Bros. being completely “night” (and I don’t mean Dark World). You could have some real fun both thematically and gameplay wise with a a world where it’s always night. Have torches lighting your way, fireballs could help illuminate gaps in illumination, you could have phosphorescent forests and plants and enemies. You could bop on a glowing koopa troopa and carry him around like a lantern throughout darker parts of stages. Could even have a level where an mean-looking moon tries to swoop down and attack you in some way. Perhaps you can have a few unique power ups to that world that let you glow or otherwise do things that make it easier to get around (but also be useful for fighting enemies). Perhaps bring a luminescent hammer brothers suit back? Perhaps a special mushroom that not only does it’s normal thing but also lets Mario/Luigi/Whoever glow in the dark? And when you go up the midway fortress you do the boss fight on the roof where whatever boss is hidden by the darkness but you could somehow reveal the moon or starlight or something to help figure out where the boss was? Mario Land 2 had a “spooky” themed world. I think “Night” themed one would work pretty well.
Or better yet, why not have ‘night’ appear in ALL the game’s outdoor levels? Why not have a night and day schedule? *Time* would pass when you either died or completed a level. And if you don’t like playing at night, you can go back to Mario’s house or Peach’s castle to sleep and have dawn appear.
What would be the point of this? Well, the game would feel more like a cohesive world. Hammer brothers and other roaming monsters could sleep during night time (allowing you to walk past them). While nighttime would be harder in the levels, some unique things could appear only at night in those levels. Maybe a special exit could only appear then. Perhaps certain monsters in those levels only come out at night. What if a power-up can only appear at night?
There is so much that can be done with 2d Mario that it is ridiculous.