According to the devs Q&A, they don't think there would be enough interest/use to warrant adding a sandbox mode where guild members could set teams, and others could fight them. Personally, my guild would use the heck out such a feature, but maybe we're just outliers. I know the forum isn't the perfect sample size, but I still think it's a pretty good determinant.
How often would you use Guild Sandbox? 55 votes
Occasionally (such as when a new team is released)
"I don't like sand. It's coarse and rough and irritating and it gets everywhere."