Hello Master Malstrom. That idea reminded me of Sonic CD which is a pretty damn fine version of Sonic. They integrated the time mechanic based upon maintaining speed and made it a challenge to control with platforming. It made for some very interesting and challenging gameplay that’s still fun to replay today. It is not easy to get the perfect ending of this game. Mario focuses on platforming and exploration but I can see how a time mechanic would work. It would probably build upon SMW’s alternate stage exits which would work pretty well for Mario.
A problem with Sonic CD was that the stages could feel very like they lasted too long the deeper you moved into the time mechanic. The alternate exits of SMW could provide a way to allow for the game still be experienced in bite-sized chunks without feeling uncomfortably long. Stages that you cleared the first time via the alternate exit in SMW wouldn’t allow you to progress normally, you still had to conquer them normally. That could stay. Changing the levels as you go back and forth in time could also work but there needs to be something else as a part of this type of world/mechanic to stop it from becoming puzzle-based glue without repeating the whole Halloween theming of the other world in SMW.
Oh, here’s my real contribution to the conversation.
Kingmaker/Invasion World: A world that takes place during an initial invasion of the Mushroom Kingdom. Since the Koopa Clan has strong magic, they can now receive the attributes of any power-up that Mario touches (in addition to making the Toads blocks). This would be different than the Tanooki Tail nonsense of that 3DS game. Grabbing a fire flower could mean turning Hammer Bros. into Fire Bros. or setting the tops of Goombas ablaze, etc..
I really loved how warped World 7 was in SMB3. It was a pipe/plant world but it thoroughly provided a challenge. You could sense the Alice in Wonderland influence the strongest here based upon how much the world really tried to destroy how you perceived everything. Even the moving platforms were more challenging because you had to think about your jumps. Why not build upon that line of thinking more:
Koopa Barracks: Maybe Mario stumbled in by mistake or meant to enter here, who knows? Somehow on your way to Bowser’s Castle you entered the place where the Koopa’s train and prepare for their wars on the Mushroom Kingdom. You encounter many if not all of the enemies here doing what they doing elsewhere but with their direct environment being more challenging/different than you normally encounter them in. Green Koopas like to walk off of platforms for some reason, may they do this because when they walk off of the platforms in the barracks they are immediately launched back on to it from its’ backside? The Paratroopas normally hang around stairs/stacks of blocks, maybe in the barracks they’re jumping around on a set of appearing and disappearing blocks? This world would place a higher challenge on those who play it but take advantage of the known properties of enemies to make the environment a bit more difficult to get through.
Why do people keep suggesting an ‘Invasion World’? EVERY WORLD HAS BEEN INVADED. That is the premise of the game! Goombas and koopa troopas don’t ‘live’ in the Mushroom Worlds. They got invaded and conquered.