I personally think it does need to be redone, but your idea of the pressure-plate door won't work. You need rights to a town to activate pressure plates, buttons, levers, ect.

 

just a wall and a doorway with some form of arrow blockage (reeds i think)

edit: nvm reeds dont block arrows anymore

 

Sadly, reeds no longer block arrows, it was fixed it 1.6.
and it wouldn't look too nice anyway.

 

How about if there is a 2-deep wall of water all around the pvp area? Water affects arrows trajectories by making them fall much faster, so then the arrows would be mostly blocked (you'd only get hit if you are in the water or veery edge)

 

yes this arena needs to be better in many ways.

people run and hide.
people shoot from the stairs.
and sometimes people tp out (dont know if that can be stopped)

 

Well you should just add glass walls or something transparent. So that players that hides wouldnt be able to run up in the stairs and hide.
otherwise, newbs will be newbs.

 

Also what about this if you enter fight your still alive and stuff then you want to leave how do you get out and you could always do /spawn or /home