Cool. Since your characters are known to travel between worlds/dimensions, the elements don't need to match up exactly. 

So, which plot would you like to try? Something on this list, or another idea?

Random ideas: 
Fetch quest
Rescue mission
Escape attempt.. Whether from a natural catastrophe or some villain.. A natural disaster could be interesting with a set of elemental characters.
"Let's be friends" - The groups are antagonistic toward each other, for unknown reasons. Maybe a misunderstanding. (this would work best as a sub-plot)