Something that has been brought up before in a previous discussion that I believe still needs to be mentioned is a Guild Battle Arena.
I know sometimes you want to match your heroes against your own guildmates just to prove who is superior or just for fun. If this does come down it could be like fight pit or Coliseum where each day you get so many attacks. However this would be the trade off no experience or anything awarded.
It would also be a good proving ground for heroes. So if you get a new hero, like recently burned one or void wrym on s3, you could test them to figure out what lines would work with them what wouldn't without having to try out on Expedition, crypt, guild war, fight pit or Coliseum. It would give people a note accurate test of heroes then expedition or crypt
This would also add more to the game. You would be having more people on and bring extended life during periods where we are waiting for either level increase or gear increase.