Current plan: have a safe stairway in the center of the spire to carry prisoners up and subsequently throw. 1/2 of the tower will house magma and related pumps, and the other half will house water and related pumps. Some residual area will be used for prisoner housing, barracks, viewing booths, etc. Large amounts of fluid logic and reservoirs should be housed underneath, positively massive pits of fluids and machinery under the bowels of the arena.
Then there's two options. 1: Porcelain bricks. I've come across a large amount of kaolinite, so I'm making bulk bricks. I think it's a nice idea, pristine white, smooth surface of death and the anvil upon which the creatures of the earth are stuck. 2: Pillars inside a pit, and then floor grates around the pillars, since a floor grate needs nearby support to function. For these, likely steel or bronze instead, or perhaps just floor bars using raw bars?
Porcelain floor: Easy to set up, allows traps to be placed along the surface and abuse water timers to trigger upright spikes.
Grates: A bit harder to set up, but not overly difficult, allows the theatrics of liquid abuse as it rises up out of the floor and then just as quickly drains back down, thus ensuring very quick cleaning of the arena and melting of all worthless junk. Iron and steel items can be dropped via lever, causing the entire floor to vanish (marksdwarf shooting range, anyone?) and drop the goodies below. Ideally, there would be a second layer of grates, allowing valuable goods to be salvaged even during a fight. This means two layers of grates above a magma cistern, and on command the magma can be raised or lowered to either layer, or higher.
Important: Grates can be smashed by building destroyers, but they should be busy with the live targets anyways. Grates also do not allow traps, though the pillar tiles can hold traps, decorative statues, or pillars.
I just talked myself into using grates, so I'll be sure and find a use for all the porcelain bricks somewhere. No shortage of options in a construction this big!
NINJA: I think I'll add a third layer for hatches, allowing magma to be optionally drained VERY low, and water pushed up in its place, but there will be some cycle delay between swapping fluids, that'll be a bit of a bugger. Both fluids will be designed to pour from the top, but the main bulk will be pressurized through the floor or the walls. For a good "walls pouring magma" the under-level can be filled to 7/7 and the upper layer will simply wash over as if it were solid floor.