Giga Bowser is a character that doesn't exist outside of Smash, so it make sense that he only apparear on FD, the stage he was born.
wow that's deep
Ganon meanwhile does come from a game, OOT in that regard, so it make sense they recreate the area where he is fought. Not to mention they already have Ganon model, Giga Bowser is Bowser FS yet it's still a boss, so it be a waste to not reuse Ganon FS model for something else.
Nobody as found a stage that ressemble 100% the one we see in the image, some said it might be the Mii stage, but they are details that aren't in the Mii stage, like the flame and the piliar.
Others said it might be Spear Piliar or N Castle, but...
The flames and stage in general don't match up.
They are piliars, but they don't match up.
I tell you, it's happening.