/r/StevenUniverse has 57919 subscribers at the time of writing. I am a moderator of the subreddit.
We ran a contest in order to pick two ad designs to submit to /r/SubredditAds: the 300x100 design voted for by users, the 300x250 design voted for by mods.
This 300x100 design, by yours truly, won the user vote. It prominently features the show's iconic Crystal Temple. I opted to use a high-contrast image from the show in order to make it as eye-catching and intriguing as possible.
This 300x250 design, by /u/isagsani, won the mod vote. It's a piece of art they made themself, and all the mods agree that it's absolutely gorgeous -- it sums up the fun familial atmosphere of the show.
Steven Universe, created by Adventure Time alumnus Rebecca Sugar, is known for appealing to adults and children alike. The show has a broad range of appeal, and our two ad designs cover this well: the 300x100 design draws people into the show's mystery, while the 300x250 design reflects its joyful spirit.
The show is about to begin a promotional event on an unprecedented scale, making this the best possible time to advertise its subreddit. Steven Universe occasionally has broadcast events called "Stevenbombs": certain weeks in which brand new episodes are aired every weekday. On /r/stevenuniverse, these have always been accompanied by a significant spike in subscribers and activity. Today, July 18, is the first day of the Summer of Steven event, consisting of new episodes every weekday for four consecutive weeks. It's equivalent to all four previous Stevenbombs put together, and will almost certainly bring interest in the show to a record high.
As a result, a good number of people may end up discovering the show for the first time, seeing it on Cartoon Network or being caught up in the inevitable boom of internet hype. Having /r/stevenuniverse ads Reddit-wide will ensure they know where to go to discuss the show or enjoy fan content. Conversely, the ads themselves might be the first some people hear of the show, and the Summer of Steven event ensures that the subreddit will be more than active enough to draw them in.
Summarised in less than 140 characters:
/r/StevenUniverse, for the Cartoon Network show. Gemstone aliens, life and death and love and birth, and peace and war on the planet Earth.