Now that's an interesting insight. Music therapy!
Brooding? Only parts of the O Fortuna chorus strike me as 'brooding'. The rest of it is so strident that it's frequently used as battle music. Surely you've seen some of the WW II movies that use O Fortuna during some big shoot-out? There are several. So in my mind 'similar' music to O Fortuna would be 'Classical Thunder' type pieces.
The "infamous" William Tell Overture by Rossini
(Hint: The Lone Ranger theme.
)
The 1812 Overture by Tchaikovsky (with cannons!
The 'Mars' movement from The Planets by Holst
(Now
there is a famous piece of battle music!)
I think the 'Jupiter' movement from this work is also very stirring. It's more on the 'positive' side like the victory celebration after the battle though -- so it's only useful if there
is a victory in this story.
Montagues and Capulets from the Opera 'Romeo and Juliet' by Prokoviev
Tchaikovsy is also supposed to have made a pretty fierce 'Montagues and Capulets' piece in his Romeo & Juliet Opera but I've never heard it.
In the Hall of the Mountain King by Edvard Grieg
The Ride of the Valkyries by Wagner
(Yet another 'infamous' piece of classical music
)
There are more. Rather than type them all out I'll point you to the URL where you an order the CD:
http://www.cduniverse.com/search/xx/...al+Thunder.htm
And then after 'classic' classical music there are things like movie and game soundtracks. You might try the Starship Troopers soundtrack. Very 'military' stuff.