Ok, this is somethings I both came across and thaught up, and after looking through the dev pages involving the plans for the arena, and turned up nothing...I like the arena very much and see it as a fully fledged offshooted mode later on down the road aimed at training the inexperienced in df.
I first shall suggest some modifications.
1. There is no way to destroy or modify the walls while in the arena, and changing stuff is only in the file, it would be helpful if at least there was a way to turn something into empty space, or ramps or whatever while in there (yes I know the function will become very useless once adventures are given the ability to dig and ect but until then).
2. The ability to contol the ambient temperature, so as to make and test freezing, boiling and other object temperature effects (like snow and boiling rock).
3. The ability to add non equipable items and such, on creatures as well as off, for the fun (mostly because I want to test if magmamen can eat beer in the arena, and and have the ability to spawn beer above magma if I want to).
Now some things I've noticed, all while inside the arena, take what you will from them.
With hacks you can make it snow and rain in the arena but cant read or change the local temperature, so snow is independant of temperature and can happen anywhere the region is right.
Drop cast obsidian walls make inside but rest cast obsidian dosent.
Fat gotten from butchering something that died in water can be laced with water, and have a water covering.
Stuff you take control of can gain titles like in adventure mode.
Melting dwarves melt alot faster when you are controling them.