I'm wondering if it's possible to merge the two styles. If we add a prompt to the new sign up template (I only now noticed that it doesn't include a prompt) and send you a ranking of preferences to the public sign ups, those who prefer exchange could proceed as usual and those who prefer challenge could regard the prompt as the challenge. The recipient would then at least get their prompt addressed. And the challenge writer would make the ranking based on the offered prompts. Does that make sense? Just a random thought.