The Outdoor Stage is literally a free space, usually used to advertise new gachas.
The Outdoor Stage is commonly used as an area to advertise brand new gachas. It is literally free space, as stated in the description of the area on the Selfy Town Map.
This area is a green field with a huge, multi-colored stage, made out of gold, white stones, purple stones, turqoise stones, and orange stones. At the top of the stage is an amethyst-like stone, with lights shining on the screen of the stage; as pictured above, the screen is advertising the Vocaloid Persona 5 gacha; this is not always up as stated before; several gachas have been advertised before (e.g. the other Vocaloid Persona gachas, the Durarara! gacha, the Black Butler gacha). Sometimes famous characters have even shown up there; however, they are only cpu's that "breathe" and are interactable (meaning when you roll the cursor over them, they say a preset phrase).
Down: Gourd Pond
Left: ChibiPet Area - Gate
Right: Central Square
Up: Game Area
Selfy Town Map Description: "Free Space"