On the west end of the bridge, add a second oni boss (with the Windy Oni monster graphic from the Moonspell DLC) and have it unlock another secret character, one like Scorej-Oni but using the Silver Wind weapon instead. 
This would add a nice symmetry to the level, introduce an interesting character with a new spin on an established mechanic (close range spam instead of screen-wide), and fit the level's description of two armies.
...If DLC can't modify base levels, I suppose Vento Sacro would do as a compromise.