For fun, when messing around with the temperature controls in the arena, I discovered a way to shape the landscape of the arena at will to what I want for testing purposes.
The method of forming rock in the arena is easy, and known, put down water then lava on top to create rock.
Removing rock is more complicated.
With the new temperature controls, it is possible to remove blocks. This is done by putting water on the block(s) that you wish to remove. Then set the temperature to a setting that will freeze the water to ice. (Absolute Zero works very well). The creation of ice on the blocks destroys the original block and makes only an ice block there. Turning the temperature back up will convert the ice to water, or sublimate the water if turned up to fire or above.
The sublimation of the water still leaves behind mud, which can be easily removed with the remove contaminants button under weather. Unfortunatly, the effect of sublimating/evaporating the water/ice instantly drains the lake and rivers, as well as lighting all exposed vegitiation ablaze.
There are some interesting side effects, for example, if the arena has gotten very messy with blood and body parts, setting the temperature to dragonfire will literaly vaporize every organic item in the arena, and light living plants ablaze, including nethercaps, which should be immune, and excluding dragons.