Since the last event night was a success, and featured a new fighting style, I thought of a new one, also featuring water. I got the idea from a combination of boat guardian concept and water battles.
The general idea is this: A large "fish tank" is made out of glass and filled with water. The bottom should be decorated with cover. Several guardians are spawned each round in it to discourage camping and also to liven things up a bit (this is why cover is necessary, otherwise RIP all contestants). Contestants will dive in with an assigned kit and fight till the last man standing.

Kit:
Pumpkin (Helmet allowing water breathing)
Leather "wetsuit" (chestplate and legs)
Wooden or stone "knife" (whichever is preferred, I find wooden swords with leather armor quite fair and fun)
Bow "speargun" with a 16-arrow clip (can be extended to 32 if many contestants)

Notes:
In water-surface PvP, bows are often very powerful as the person being targeted cannot move fast to dodge. However, arrows suffer a lot of drag underwater, effectively shortening their range and making them less OP, but still an effective strategic weapon.
The pumpkin gives an accurate "helmet" look.
Depth Strider boots might be implemented, as "flippers".
There is no need to name items, names are there just for relatability.

As Link said, we already have an underwater PvP arena. However, it has never been used for an official event, and something could definitely be done with the idea! The availability of Depth Strider in 1.8 definitely gives a new edge to this.

If you're willing to put some work into the idea, you could setup the event yourself and send me a formal description, so I could put it on the website's news whenever we're ready to move forward. Player initiatives are welcomed, and it's fun to see what others can come up with! The same applies to other people who might have event ideass.

The existing arena could be modified (if AshaLina gives her permission) if you want to work with it, or a new one could be built for this purpose.