Rather than suggest specific ideas, I'll give you some general suggestions.
When I start my dios I spend a lot of time trying to visualize a story that can be told with a "picture" and under 6 words. I also try to determine the mood of the piece - in fact this is my first step.
Try to ask yourself the following questions:
1. What mood do I want the diorama to have ? (ie: sombre, frantic, etc)
2. What do I want to be the focal point ? (ie: is it a model in a setting, or an entire scene)
3. What sort of theme do I want ? (eg: action, details, scenic, humourous, etc)
armed with these answers, I usually find an idea comes to me. In your case, you're limiting yourself up-front by already determining "key players" in your dio. But you can work with that - put them out of your head, and think about the questions.
Once you have the answers, think about the models - don't try to think of a scene, just mull over the 3 answers and the 2 models.
I absolutely guarantee you that within 48hrs, you'll come up with a whole bundle of ideas. Once you have them, then everything will just fall into place.
Hope this helps you