Theory: Wild Valley's Location

Abstract
Wild Valley from Mario Superstar Sports is past Bumpsy Plains from Mario & Luigi: Bowser's Inside Story, in the Koopa Kingdom.

Arguments

 * Argument 1: As seen in Yoshi’s Island's last world map (World 6), happening in the Koopa Kingdom, pine trees and vast valleys are seen all over it, just like in Wild Valley. It also features dead trees in its background, which are frequent in the Koopa Kingdom landscape.


 * Argument 2: The landscape of Wild Valley tends to show that the climate is quite arid and hot, just like seen in the various representations of the Koopa Kingdom, while the yellow grass tends to look like the grass in Bumpsy Plains.


 * Argument 3: The destroyed structures like old deteriorated bridges and houses could show the consequences of the many wars involving the Koopa Kingdom, as they could have been destroyed forts, for instance.


 * Argument 4: The presence of what appears to be the coast of a body of water doesn’t contradict the Koopa Kingdom placement, as the kingdom is shown to have bodies of water, just like in World 8 of New Super Mario Bros.