What we saw at the end of Book 9 was just a prank.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.
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.
Every villain that's not dead will return.So Tippy Tinkletrousers has returned, but died at the end of Book 10, and the teaser stated that the next book will have a return of 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.