I like the idea of having a general theme, and then presenting what you've built at the end somehow. I'd rather not do a hackathon that required we use a specific API or product, or otherwise restrict what you work on. Instead, make the hackathon about building things along a certain theme ("music", "transportation", "education", etc).
Also, I may be biased as a hardware engineer, but I only consider hackathons that welcome both software and hardware projects.
Also, I may be biased as a hardware engineer, but I only consider hackathons that welcome both software and hardware projects.