We can conclude one thing, though. If Xerneas appears as a stage hazard, the stage is sunny, or at least sometimes is. That's not exactly a rare trait for an area, but it might allow us to disqualify certain places. Obviously indoor areas, but we can probably eliminate any areas in XY that are shrouded in different weather conditions, like Snowbelle City. That's just an example, I doubt Snowbelle would ever actually be a Smash Bros stage.
Edit: (XY Spoilers)
Going by that picture, Geosenge Town appears to be sunny even during the actual plotstuff. Couldn't remember for sure.