Even when a cartoon is lucky enough to go on for a few years, that doesn't mean that the writers get to introduce everything that they wanted to do. Sometimes, a season ended on a cliffhanger and didn't get a chance to reveal the twist. Other times, a character seemed to be forgotten about, sometimes because their original fate had to be scrapped at the last second. And when a cartoon is based on a comic or book series, it's not unheard of for the show to end before it gets to the last story arcs.
This is especially common for cartoons that have had multiple incarnations over the years, since, rather than finish the plot of one spin-off before it gets resolved, it just goes on to another show, or even brings back the parent show.
10 Avatar: Only The Comics Remember Princess Ursa
Zuko's mother, Princess Ursa, left quite an impression on fans, being one of the more sympathetic characters of the series' Fire Nation. She agreed to be banished to protect her children, having murdered Azulon to protect her son. Her ultimate fate, however, ended up being cut from the series finale.
Fortunately, the story was eventually resolved in a spin-off graphic novel, The Search.
9 Scooby-Doo: Even A Movie Years Later Didn't Capture The 13th Ghost
In The 13 Ghosts Of Scooby-Doo, Scooby accidentally unleashes thirteen evil ghosts onto the world and has to recapture them, helped by a Warlock known as Vincent Van Ghoul, who was portrayed by the actor Vincent Price. Interestingly, the series employed ideas later incarnations would be noted for, like having the monsters be real and giving Daphne a more proactive role, with her as the de-facto leader of the gang.
The show notably ended with only 12 of the ghosts captured. A follow-up film, Scooby-Doo! and the Curse of the 13th Ghost, was made to address this years later, it technically still ends with the 13th ghost uncaptured, leaving its fate somewhat ambiguous.
8 Codename: Kids Next Door: There Was Going To Be A Spin-Off
In this long-running Cartoon Network show, the main character is essentially shipped off to space and the show more or less ends. A finale involving a villain trying to find his whereabouts years later, but ends with his friends keeping his secret. A spin-off had been teased focusing on the still-young kids getting involved with galactic conflicts, but not much came of it.
On a related note, towards the end of the series, a group of recurring villains called the Delightful Children From Down The Lane, were revealed to be brainwashed KND agents. The series notably ends without resolving anyway of changing them back.
7 Duckman: The One Time They Ended On A Cliffhanger Backfired
For most of the series, Duckman's beloved wife, Beatrice, was a posthumous character only seen in flashbacks. The series toyed with the idea of her being alive but revealed it was actually her identical sister. In fact, the character was originally intended to be alive from the start, yet was replaced by another identical sister. However, the series finale revealed that she was indeed alive after all. Although her exact story was never explained, she revealed another character, Cornfed, knew the truth and hid it from Duckman for undisclosed reasons.
While the series had recurring storylines before, like the grandmother being replaced by an imposter, this was all the more surprising because it was one of the few times the show ended a season on a cliffhanger. While this is the type of show that would end on a cliffhanger as a joke, allegedly the showrunners really were under the impression the show was being renewed.
6 Gravity Falls: What Even Is Robbie?
Dipper's teenage rival, Robbie, seemed to have a deeper connection to the paranormal than he lets on in Gravity Falls. One episode involves Wendy and her friends being targeted by a pair of ghosts at a haunted convenience store, each trapping them or horribly mutating them in some form. Strangely, Robbie is spared their wrath with no explanation. While the ghosts are appeased before doing anything to Wendy, Robbie literally seems to disappear during the conflict, especially since he didn't seem to know how Dipper calmed the ghosts.
A later episode also revealed that Robbie was manipulating Wendy with secret messages in his music, without even explaining how or if he was aware of what he was doing. This has all lead to fans speculating he's some supernatural being himself, but the end of the series never really explored this either.
5 The Amazing World of Gumball: Fans At Least Got Clip Shows
Clocking in at six seasons, the series had a surprising run but didn't seem to have much time to resolve the ending. The final episodes hint at the show's ending, treated as some horrific event, finally culminating with one of the villains attempting to launch the cast into the real world.
A void appears towards the end and absorbs the villain and the show pretty much ends there. It was previously hinted that the series would show other characters at least trying to escape their fate, but that was never really shown. Fans speculate a television-movie was supposed to give the series a more conclusive ending, yet the closest fans got were a few clip-show specials that treated the characters as perfectly fine.
4 Spider-Man: The Hero Gets This A Lot
The Spectacular Spider-Man more or less ended production when Disney got the rights to the character. Had the series, continued, more villains would have been introduced and Peter's romance with Mary Jane would have been explored. Despite speculation, while Gwen Stacy ended the series alive, the showrunners confirmed there was never any intention to kill her off.
Admittedly, this tends to happen to Spider-Man a lot. A similar fate also befell Spiderman: Unlimited and MTV Spider-Man, the last of which ended with a character inspired by Gwen Stacy, "Indy," stuck in a coma.
3 Teen Titans: Who Is Red X?
In the 2000s Teen Titans cartoon, one early episode features Robin donning a disguise as a villain called "Red X." However, later episodes would introduce Red X as a completely separate character, who possibly got their hands on Robin's costume, without much of an explanation how. A New Teen Titans short eventually poked fun at this by having the Titans finally capturing and unmasking him, only for it to be a decoy. For what it's worth, Beast Boy thinks he's Jason Todd.
On a related note, the series also ended before a season could be written revolving around Starfire, considering the first four revolved around Robin, Beast Boy, Cyborg, and Raven, respectively.
2 W.I.T.C.H.: They Waited Until The Last Second To Introduce The New Guy
This adaptation of the Italian comics, in which five girls are given powers over the elements to protect magical worlds, only lasted two seasons, despite having around a decade of storylines from the comics to adapt.
While the series more or less ended on a happy note, with the main villains defeated, and with a narration explaining the characters' fates. The finale also introduced characters that would have become more prominent had the later comics been adapted, even ending with the introduction of Raphael Sylla.
1 Winx Club: Even The Parent Show Forgot Baba Yaga
World of Winx, a Netflix spin-off of the Italian cartoon, Winx Club, revolved around the fairies on Earth who are protecting the World of Dreams. The final episode revealed a recurring character was the legendary witch Baba Yaga in disguise, who planned to revive her coven to take over the Earth. However, the series ended there.
Interestingly, the parent show would prove to outlive the spin-off but did not attempt to resolve the Baba Yaga storyline into the main series.