There are various new 2019 Invader Zim fanfiction stories. Some might focus on new adventures of Zim and Dib on Earth. For example, there could be a story where Zim discovers a new way to try and take over the Earth that involves some strange new technology he's created. Another might be about Dib finally getting proof of Zim's alien nature in a really unexpected way.
There could be a new Irken who is sent to Earth to spy on Zim and make sure he is doing his job properly. This new character might have a different appearance and personality compared to Zim, perhaps being more calculating and less impulsive.
Well, it could be about a space girl who encounters Invader Zim. Maybe she tries to stop his evil plans on Earth. She might use her special powers from outer space to outwit Zim and his various gadgets.
One popular Invader Zim girl fanfic might be those that focus on a female character's adventures in the strange world of Invader Zim. For example, a story where a girl discovers Zim's true identity and has to decide whether to expose him or help him in his bumbling attempts at world domination.
Often, there's a theme of self - discovery. As Female Zim is a different take on the character, she might be on a journey to figure out who she is in this new form. Also, the theme of adaptation is common. Just like in the original where Zim has to adapt to Earth, Female Zim may have her own unique ways of adapting, whether it's to Earth's environment or to the reactions of the other characters around her. And there's also the theme of power. How does being female change Zim's perception and use of power? This is something that many fanfictions like to play with.
Here's a more detailed fanfiction idea. In a world where the Powerpuff Girls have been unable to stop a powerful new villain, they seek help from Dib, who then brings them to face Zim. Zim, thinking he can use this situation to his advantage, offers to help them. But in reality, he has his own plans to take over both Townsville and the Earth. As they work together, the Powerpuff Girls start to notice Zim's true intentions. Bubbles tries to befriend Gir to get more information, while Blossom tries to keep Zim in check. Buttercup is more skeptical and constantly on guard. Meanwhile, Dib is trying to document everything for his own records, hoping to finally prove Zim is an alien once and for all.