Stage: Area Zero
Area Zero, for those that have not played Scarlet and Violet, is the final area in the game. In the Great Crater of Paldea, you have Area Zero. This bizarre, almost Eden like location is home to several Pokemon, but most notably has the Paradox pokemon. Scarlet and Violet have past and future pokemon respectively lurking in Area Zero, brought here by experiments with time. Compared to the rest of the game, your first visit to Area Zero is far more linear, a slow trek to the depths of Area Zero and the Zero Laboratory. Koraidon/Miraidon are reluctant to even come out of their pokeball due to their trauma here. You are forced to take the linear path to the depths with your friends and learn how this twisted Eden could potentially ruin the rest of the region.
I think this is the perfect stage for Scarlet and Violet. Spear Pillar, Unova Pokemon League, and Kalos'Pokemon Leagues show they are not too scared of doing a late game area for a Pokemon game. I think you could either do it like Northern Cave, where you drift through the stage from the top to the depths, or you could just choose one location to set the stage in. Maybe right outside the Zero Laboratory, maybe right near the entrance? You could do whatever. I think it would also be cool to see Paradox Pokemon show up. I think the ones I would go with are Great Tusk, Slither Wing, and Roaring Moon to represent the past and Iron Treads, Iron Moth, and Iron Valiant to represent the future. You could also have Koraidon and Miraidon show up as Pokeball Pokemon or be present on this stage instead as stage hazards.
Music
Scarlet and Violet have an insane soundtrack. Hats off to Toby Fox and the rest of the sound team. Here are a few songs I think would be great picks.
Just so many possible bops to go with. My personal remix pics would be a medley of all four Province's wild battle themes, an Area Zero Medley, and a Team Star Boss remix.