[[WMG: What we saw at the end of Book 9 was just a prank.]]
[[spoiler:George and Harold may be giant zombie nerds now, but they're still pranksters at heart. The "red squishy stain" left behind was actually some ketchup covering Tippy.]]
* It appears I was partially right: [[spoiler:ketchup was indeed involved, but George and Harold's personalities had nothing to do with Tippy's survival]].
[[WMG: There's still one Tippy Tinkletrousers left.]]
As of the end of ''Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers'', there were four Tippies in existence -- the original Tippy (who may or may not have escaped from jail by now, depending on whether he was still tasked to build the Warden Gordon Bordon Shmorden statue), Tiny Tippy/Supa Mega Tippy (who was created during Tippy's original trip to George and Harold's Kindergarten days), Big Tippy (who was created when Tippy went back to fix the timeline), and Slightly Younger Tiny Tippy (who was created when Tiny Tippy was sent to see where George and Harold disappeared to). The three Tippies created through time travel died during the events of the book, leaving the original Tippy (who was simply not involved with the story) still at large.
* Actually, the original Tippy was Big Tippy. He went back to when George and Harold were in Kindergarten by accident, then went forward, which caused the apocalyptic scene, then he went back to when George and Harold were in Kindergarten to prevent that from happening, and that's what caused the first two Tippys. So yeah, they're all dead.
** Possibly confirmed: at the beginning of the following book, in the background of the scene where Captain Underpants, George, and Harold return to the future, we see Tippy in his Robot-Pants bumping into the school.
[[WMG: Every villain that's not dead will return.]]
So Tippy Tinkletrousers has returned, but [[spoiler: died at the end of Book 10]], and both the ending and the teaser stated that the next book will have a return of [[spoiler: Melvin Sneedly and the Turbo-Toilet 2000]]. Every villain who isn't dead might return for another book until the very last book, which might feature a return of Dr. Diaper.
[[WMG: Mr. Meaner the Gym Teacher will be the next Villain in the book series]]
He will eventually read all of George and Harold's Captain Underpants comics and realize they ALL have jokes that involve the Gym Teacher getting some sort of horrible fate and no one caring, so he would attack George and Harold out of retaliation for the "death to the Gym Teacher" jokes.
* Personally, I envision a return of the [[HypnoRay 3D Hypno Ring]]. Yes, I realize it's unlikely, but stranger things have happened... I guess?
* Possibly confirmed. At the end of the tenth book you can see Mr.Meaner with stink lines coming from him,possibly meaning that he might be Sir.Stinks Alot?
[[WMG: Harold might be bi rather than StraightGay, due to having a crush on a girl]]
While Harold [[spoiler: (or at least Yesterday Harold)]] does grow up and is happily married to a man, we do get hints he might've crushed on a girl. In Ook and Glug, there is a subplot where the granddaughter of the OldMaster said cave-kids (whom are based on George and Harold) are learning from ends up mutually attracted to Glug (the one who is based off of Harold.) They even end up married in the end when she stays with him in the past. Considering that George and Harold started taking martial art classes (as said in the back of the book) and the knack of basing their fictional characters off of the people they know in their life, it's likely that said OldMaster and his granddaughter also were inspired by George and Harold's sensei and his granddaughter. Furthermore, the amount of practical wisdom he dispenses and how he seems to be one of the ''very few'' [[ReasonableAuthorityFigure reasonable and good-hearted adults]] we see portrayed by George and Harold, (or in general in all of the Captain Underpants series, spin-offs included,) it would make sense that he was based off of George and Harold's own sensei. Ergo, Harold having a crush on the granddaughter would explain why romance was included in this graphic novel in the first place and played straight rather than as a parody, especially with her Expy falling for Harold's Expy, Glug. Harold has a crush on her and expresses those feelings here while George is no doubt encouraging his friend and having his back.
* Alternately, he could've just been in the closet at the time, but still is important as it served as his first encounters with romance.
* He could have had a crush on one of the cheerleaders.