I know some of these have likely been suggested before, but I'm still going to include them in the list!
Remove Creature
Difficulty: easy
Let us remove creatures already placed in the arena. Useful for when we accidentally place the wrong creature, or the right creature, but on the wrong team, ect.
Change Alignment
Difficulty: easy
We should be able to change the team a given creature is on. Besides being useful when we accidentally forget to set the team of the dragon so that it actually fights the army of kobolds, it could provide much amusement by letting us create traitors from the ranks of an already fighting army.
Change Equipment
Difficulty: moderate
Let us change the equipment of a creature already in the arena.
Heal/Hurt
Difficulty: easy/moderate
Ever wanted to see how well a dragon with one eye and no legs fights against a random dwarven peasant? Also good for arena tournaments, where we might want to heal the contestants between rounds.
Move Creature
Difficulty: easy
Good for tournaments, as well as placing a creature in an otherwise inaccessible location.
Unlimited Ammo
Difficulty: moderate/hard
How long does it take for an unskilled kobold archer to kill a hydra that's trapped in a pit? I don't know, they keep running out of bolts! :-(
Edit Creature
Difficulty: hard
What happens when you give a goblin an extra head? Or give the CAN_LEARN tag to a bronze colossus?
Equipment Profiles
Difficulty: moderate
Axeman: 1 suit of iron armor, 1 iron axe, 1 iron shield. Ect.
Save Game
Difficulty: moderate
Being able to save an arena battle in progress could be very useful for sharing battle setups, best 2 out of 3 tournaments, and so forth.