((If your still having trouble thinking of ways to start it, just bear in mind it's an equilibrium/disruption setting. This means that everything is fine up to a certain point until something tragic or something terribly different happens to force the characters into a new setting/change their lifestyles completely.. At least, this is what I gather from the setting you described, I never actually watched the show. I'm not sure exactly how the dragon will come into play, but it gives us options such as he attacks the bus, a wormhole opens beneath the bus and the whole thing plus the dragon fly through it sending them into the "alternate future". Another way is that, being a dragon, it probably would have a much longer lifespan, so it winds up becoming powerful and somewhat almighty in the time the bus is in the wormhole, sending the passengers into a future that is so vastly different they find little reason and know naught about what to do. These are just ideas I have, mind you, feel free to turn them down.))