With the Hey Arnold movie actually coming out and wrapping up the series, let's talk about all the canceled comics & cartoons that wound up getting an ending. Anything that at least had people in charge say how it was supposed to end is worth noting as well.
Pirates of Dark Water is one of the most infamous examples. Cartoon Network apparently even did a promo with a joke that they recorded over the ending with home video. Despite how well known the lack of ending is, surprisingly few know that the video games (The Genesis and SNES games are entirely different) are actually pretty good and the Genesis game has an actual ending for the series (the SNES one just ends on a non-ending).
Power Pack Holiday Special was released a while after Power Pack was canceled because a new creative team ran it into the ground with really crappy art (Seriously. A girl who was barely in the double digit age now looks older than her mom in the same panel) and weird direction (Alex transforms into a horse!). In the Holiday Special the original creators manage to undo the weird changes in a natural way and give the characters a conclusion.