The catch with having 5^4 (625) possibilities, is that its a bit hard to analyze trends with so many choices. Of course, from the standpoint of someone who isn't approaching this as a social scientist - that variety would probably attract far more interest than a binary choice. I rather like the idea.
We can't really confirm people are rolling randomly. Of course 24 hour games are on the honor system so that may not be a problem. However it probably would be a bit more fun to write as you like than randomly.
There are still 30 unused prompts from the first contest we ran. We could reuse these with a few additions for another go at the random idea challenge. Perhaps we could start a monthly thread that lists two or three and encourage people to write a quick game or a story each month. Not judged per-say, but something to get people to create more or think a bit.
Or we could go for the unofficial William Burroughs technique contest. Collect some game ideas or short stories, cut them up, cast them into the air, and collect them as they fall.
However, I will say that I'm busy for most of August and October, so at leas for me it would be be best to wait until later.