The next episode in Japan is called:
"The Three Animal Appearance! The Referee School Island!"
and is going to be aired on the 2nd of September.
From the pictures shown it look as if this episode is about Ivysaur, Charmeleon and Wartortle. Maybe it's like "Prof. Oak & Prof. Birch! Secret Base Battle!" as it is showing the 3 Kanto starters in battle, but in this one it is showing the 2nd stage of the Kanto starters. Maybe there will be another episode with Venusaur, Blastoise and Charizard but maybe not because there is an episode with those 3 in already ("One Trick Phoney!").
What do you think the episode will be about?