I love this idea! These games are a great way to bring a "community feel" to the server.
Be sure to include leader boards if this is implemented! This is a great way to do a "contest" where you essentially are rewarding your players just for playing.
I use to play a game called dawn of dragons. When you would hit the world boss during the raid you would have a chance to receive loot based on how much damage you had just hit for, this way you can scale the items that are being given out to how hard the players are hitting.
It would be great to see this campaign timed style, where you attack for 3 minutes or until you die and you get however many tries(say 3-10). If you don't die you can use your same heroes, similar to expedition.
It would help break up the monotony of the same contests we see week after week. You could have different world bosses monthly. It even looks like there is a screenshot on the google app page that depicts this!
Although I am pretty sure this is just how the last stage of campaign must of looked in beta. LoL.