Captain Underpants and the Tyrannical Retaliation of the Turbo Toilet 2000, is an upcoming book in the Captain Underpants series. It was announced to be released on August 26th, 2014.
According to the teaser in the end of the 10th book, the Turbo Toilet 2000 had destroyed the Robo-Plunger and rebuilt it into a spacecraft. Melvin Sneedly has been transformed into a time-traveling squid android thing.
Trivia Spoiler: It may be possible that the Turbo-Toilet 2000 destroyed the Incredible Robo-Plunger and used parts from it to build a ship to get back home because it shows the Plunger in the teaser.