I'm really intrigued by the idea of massive team-based events.
Here are a few ideas that come to mind:
-a ffa team deathmatch combination (think hypixel's the pit, but with teams)
-a giant team death match, 50/50 split of the people
-missile wars (that custom map originally created by sethbling)
1. Capture the Flag
-Two teams split evenly, every team member will get to choose between 3 classes (each class having a different kit, for example ranger/archer class would have a slightly stronger bow with better enchants). Maybe for classes there could be Ranger (Infinity Punch Bow), Warrior (Better sharpness + Fire Aspect sword), Thief (Has speed 1/2 OR has Depth striders (If adding rivers/lakes/water sources to the arena)). Another cool addition would be to add the certain PvP items we can get in skyblocks, like smoke bombs.
-Objective is to defend your flag while trying to get the other teams flag. Once you steal the other teams flag though, you will be given debuffs like nausea or blindness or your movement speed will be reduced from 100% to 60% or something similar.
1. Twisted Bingo
-Bingo, but with a twist. If it isn't too hard maybe you could add the Randomization mod and make the bingo list easier. What the Randomization mod does is randomize every block drop. Meaning you could punch out a log from a tree, but get gunpowder.
-For this event, you could add a timer and whoever has the most of their bingo sheet filled out, wins.