Those ideas sound good, maybe make a larger spawn with ramps up and down, since anyone can enter them.

Pros: Unique lighting, and a unique style, but not at all hard to see, this makes for an interesting map. I like the idea of the trap fall, in that you have to be careful or risk being useless to your team.
Cons: The trapfall will start to piss people off pretty fast. Having fall damage in arena is annoying enough, but not being able to get back up might make people leave servers.
Also - a good Sniper or two will rip through the entire enemy team, or a good Soldier/Pyro can knock them all off, essentially putting them out of the game.
Improvements: Ramps. I know it sort of takes away from the spirit of the bridge, but being able to flank your enemy is vital in arena. Maybe make there be a fence directly in front of each spawn door, so it's not just a mass of spawnkills.
Notes: Very simple, and nice to look at, but arena needs a place for all classes and more than one class is just a walking target here.