The same way everything is invented.
Take an existing format, break it down to its core principles and concepts, iterate and break down the concept further into its building blocks of meaning and meta.
Keep stripping away the conceptual into the mathematical framework if possible.
ie emotion is apploed psychology, which is applied biology, which is is applied physics, which is applied mathematics, etc.
Van Gogh took the current trend of gothic realism and broke it down to emotional attachment and investment, and instead of dark somber tones to portray life as it was real, used bright, often caucophaunic color schemes to portray it as a dream, with brush strokes to match.
Ansel Adams took the current standard of landscape painting, broke it down into what it was, Landscapes and Focus, and used large format silver nitrate plates capable of capturing exquisite detail and miniscule aperture, F16 or tinier, to achieve massive depth of field.
He found it worked, he found people liked it, and so developed his identity around it.
Selfie photography is a natural consequence of applied egocentrism and social anthropological interaction with near instantaneous feedback. Also humans like mirrors and looking at other humans to compare themselves, and normal portrait photography is just copying what people already did with paints for a millenia. Any scene done on canvas can almost always nearly be done with cameras + a bit of special fx.
Just...look at whats been done, and discover what it _isnt_ being done based off this.