A mode where you can put your toons to use to try out new synergies. Set up your opposing team, no energy required, no rewards, just for fun! So many times a day I log into the game and spend a few...
Moving to Feedback: Guilds (There are a lot of posts asking for a sandbox mode in Feedback: Guilds, and while this isn't quite the same, it is close enough that it should be with those posts for dev review)