Some things I find helpful to have handy in your tack trunk:
Jumping boots (front and hind)
Spurs (4 kinds: short bally, long bally, long pointy, rowely[3 rowels on side and one on the end])
Extra halters!!! Your horse will break them when you least expect it!
Martingale stoppers, rubbers, rings whatever
Back up martingale (these tend to break too )
Extra bits (to many to list)
Hoof dressing (the condition stuff)
Towels and sponges
Extra saddle pad(s)
Your grooming kit aka ALOT of stuff ex. Brushes, cactus cloth, hoof pick
LOTS AND LOTS OF EXTRA BELL BOOTS!
Shipping boots (if you can stuff them in there)
Neatsfoot oil w/ applicator brush wrapped in plastic wrap
Dirt (not intentionally there )
Already assuming other things outside your trunk:
Blankets (to many to list )
More bell boots
Tack (Bridle, saddle, schooling bridle, schooling saddle, girths)
I am very sure I could write a never ending book on stuff in my tack trunk lol. So whats in your Tack Trunk?