When a game revealed at the Xbox E3 conference is actually on Nintendo Switch.
My predictions, roughly 24 hours before the event. Smash (duh), Animal Crossing, Fire Emblem. Splatoon 2 and Xenoblade Chronicles 2 DLC. Yoshi I think will be on standby for potential internal delays, while Pokémon core will be Nintendo's main title for E3 2019. For anything else, I'm just waiting to be amazed.