Submitting prompts does not obligate you to later claim prompts. Cross-over prompts are welcome, particularly those that cross the Discworld with the Potterverse. There are no content restrictions in the SSHG Prompt Fest, so you may offer (het and gen) prompts of any kind, as long as their focus is on the relationship between Severus Snape and Hermione Granger.
New to the fest? Read its rules, schedule, and submission guidelines (scroll down to find them), and then direct any questions that you may have to sshg.promptfest [at] gmail [dot] com.
Before submitting prompts, remember to log in to LJ as a member-watcher of the sshg_promptfest.
Once all that's done, you'll be ready to help inspire creators to gift us with new, stand-alone SSHG fan works!