PvP works too, along with PvE, I'm accepting all ideas
How about something that gets people to go places they normally wouldn't go to. Notable example being Zombie's event.
Maybe hide special blocks all over Kanto and Johto and first person to find them all gets a prize. Eg. Hide a predefined number of, lets say enderportalframe blocks, in obscure parts of the regions (maybe whirl islands or other places like that; atop tin tower) and players mail admins when they found them.
Another idea I had while writing this:
Needle in a HaystackAdmin clears out lets say a 200x200 area down to bedrock, put a
single block of either stone or dirt and protect it with world guard. Then fill in the rest of the area with stone or dirt (same block as the protected one) and players must mine/dig to find the protected block. First player to find it gets a prize.
May edit this if I have more ideas later.