Bowser's Castle
Above is a picture of the layout - though you should note that this
would, in fact, be a 3D stage using up-to-date graphics. I just used SMB's sprites since I'm not able to draw that.
This stage would take place on the exterior of Bowser's Castle, with a magmatic volcano background and the rest of the castle - including a massive stone statue of Bowser - looming in the middle distance. As you can see, the main stage is composed of two solid towers, with a bridge connecting them. The bridge is thick and iron, so you can't drop down through it - though if you somehow end up under it, you can jump back up through it. Above this, a soft platform that starts in the center slowly drifts horizontally across the stage.
There are a few gimmicks to this stage. One is based on the New Super Mario Bros. games. A volcano in the background will erupt and cause small meteors to fall slowly down from the top of the screen. They can be dodged easily, but doing so makes the battle a little more complicated.
The other gimmick is based on the original Super Mario Bros. Periodically, an axe will appear on one side of the stage, next to the bridge. If you hit it, the bridge will collapse, making the stage into two separate platforms for a while. Eventually, a new bridge will extend from the sides of the tower.
These gimmicks can't occur simultaneously. The volcano will never erupt while the axe is there or the bridge is down, and the axe will never appear while meteors are falling.
There's one more unusual thing about this stage - the lava. Effectively, as long as you're below the lava, it'll gradually increase your percent with fire damage (though it won't cause you to flinch). This won't matter much, since if you're down there you're basically dead anyway, but it isn't Bowser's Castle without lava.
When hazards are off, the volcano will never erupt, axes will never appear, and the lava won't deal any damage. Effectively, it becomes a wide stage with one slowly drifting platform.
Overall, the name of the game on Bowser's Castle is adapting to the changes in the fight, all of which are slow and highly telegraphed, but force you to change up your strategy and movement to deal with them. I think this take on Bowser's Castle has plenty to set it apart while still feeling like one of the most foundational places in gaming.