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" sa pamamagitan ng 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 you at all. I can do this on my own," Dib sinabi pulling away. "I'm on my own. I don't need your help, Zim."
"Dib, can't you see? You don't know what you're getting into. You don't know what you're fighting," Zim said. "Don't do this, Dib. You 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 you 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 madami ideas and a beginning! Again, I'll give props!