So I thought of a cute little trailer if Geno is revealed:
The opening scene, instead of being CG, is live-action. It shows a boy, with his face hidden, playing with dolls of Mario, Luigi, Yoshi, Peach, and Bowser. Offscreen, the mom calls the boy for dinner but he protests and says “In a minute, Mom!” She angrily repeats herself one more time. The boy sighs, picks up Mario to look at it, and walks away.
When the boy walks out the room, the dolls of the previously named characters start waking up! Mario and the gang begin running and jumping around the room while Bowser trips and falls on his face. Rubbing his nose, he looks up find them drawn to a blue box. Looking up, they see a falling, glimmering star from down from the ceiling and into the toy box. They look uncertainly at each other when, suddenly, a light explodes out from the chest which knocks them off the box and onto the floor. Mario rubs his head and looks up as, ascending in an almost holy manner is, Geno. Cue Geno splash.
Mario, Peach, Bowser, and Yoshi all look excited to see their old friend while Luigi is left in confusion of who this is.