Pokemon has revealed the Worlds 2019 Pikachu plush in a series of Tweets. Its motif appears to be based on paleontology, museums, and field research, which means Worlds will have the same theme.
Update: TPCi has revealed the artwork for the event. “Every year, special artwork is created for the Pokémon World Championships, often inspired by the location where the event is held. This year’s artwork celebrates exploration and draws inspiration from the world-class museums that can be found in Washington, DC.”