Maybe something like: Zim and Dr. Wakeman both have machines that go haywire simultaneously in their respective dimensions, bridging them. Zim begins trying to figure out how to fix it on until Zim accidentally calls the rest of the Irken, with the Tallest taking interest in Earth now that it contains a gateway to another dimension. Jenny then decides to investigate the bridge and discovers the IZ Earth and runs into Dib, who initially thinks she's another alien trying to invade Earth. Once the two figure out what's going on they then have to juggle the fate of two Earths with both the Irken and Cluster invading full force. Maybe there could be some sort of ark with Zim where once the rest of the Irken show up, he gets pushed to the side and replaced and tries to prove that he can do everything himself to impress the tallest?

I haven't seen Enter The Florpus besides the memes here and a couple of spoiler free reviews so I don't know if this would conflict with that, but hey, fanfics, am I right?

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When you think about it a MLAATR and IZ crossover wouldn't be too hard to write for when you consider the backstories of both characters: Jenny is a robot designed to defend Earth from alien invasion and Zim is an alien invader hellbent on conquering Earth, they literally would fit together like Yin and Yang.

The main conflict with a MLAATR/IZ crossover would be the setting: MLAATR is a hopeful, almost utpoian look at the future, while IZ is a sarcastically pessimistic and dystopian look at the future. Maybe they could do something similar to The Jimmy Timmy Power Hour with dimension-hopping and multiple universes.

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