I think her pose would work great for being up on that ledge/balcony. It is a large base, but that just means you should use color and light/dark to direct the viewer's focus to the figure. I'd try some preshading with the airbrush and really focus the light on where you're placing the figure. Using color or lack of color is another way to guide the eye to where you want it.
If you want to add more figures that's fine too. Although you need to decide what story you want to tell. With Joan looking up, placing one or two bad guy below her turns it into an ambush scene. Or you could have it swarming with bad guys (sort of like Jarhead's Last Light piece) and then the story becomes a last stand with Joan praying for the strength to defeat her overwhelming foes. Are their any flying figures? You could stretch the scene vertically even more, place a figure above and in front of her for a duel scene.