Not to hint at where this is going too much, but just figure that from this point, you're going to need to determine what kind of backgrounds you're going to want to have, so that your flipbook looks more complete.
Think about the setting of your environment. Also, remember that in flipbooks, there is no such thing as "background." Everything is an object on a different layer, and the layer that is the furthest back should be drawn last, unless it interacts with layers that are above it. If that's the case, better shut your door and turn off your phone because those can be very difficult to compose.
Flipbook composition is like writing a symphony. You want your tubas to do the same (or at least a similar thing) to what your clarinets are doing, while keeping their characteristics apart. That's helpful, right? I'm only trying to help.
Here's another example of something that you might consider to be analogous to a "background theme." In the
Flarfball cartoons, note that the trampoline and the grass stay essentially stationary throughout. That doesn't look that bad, and because it's fairly incidental that it is a "fixed" object, that kind of makes philosophically the background. In this case, you can draw it first because it's made even simpler. It's technically in the same plane as all the other objects, unless you want one of your characters to go in front of it.
Remember that even a trampoline can become an animated object in a flipbook. It remains stationary because you draw it the same on every page. If you want it to get bigger, that could just mean that you are zooming in. If it walks away, that's another issue.
Hills are background,
ramps are background. Whatever you want to consider your setting could be considered your background, and it's good to have a pallete of drawings that could be considered background objects (things that are technically inanimate, or incapable of moving without interaction from other animate objects).
