I NEED help with my fanfiction! Please help! I'll give props!

Okay, so I have an idea for this TOTALLY EPIC fanfic where Zim and Dib find out they're half-brothers. The problem is... I have almost no ideas! This is literally all I have: Zim and Dib find out they're brothers and they have to go to battle against... someone. I was thinking maybe the Massive's under attack and they call in Zim and Dib...? Here's an exerpt. Oh, and I forgot to mention, I got the idea for this fanfic based on "It Ends Tonight" da The All American Rejects, so... just wanted to say that. XD It's just a great song. Anyway...:
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"I don't want to depend on te at all. I can do this on my own," Dib detto pulling away. "I'm on my own. I don't need your help, Zim."
"Dib, can't te see? te don't know what you're getting into. te don't know what you're fighting," Zim said. "Don't do this, Dib. te don't know what you're doing."
"I'm on my own, Zim."
Zim looked at Dib. Dib could see true worry in Zim's ruby eyes.
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So, in case te weren't listening, this isn't a ZaDr! I hate ZaDr! Zim and Dib now know they're brothers and Zim has brotherly feelings towards Dib. I need più ideas and a beginning! Again, I'll give props!