In the past it's all been together, but you can say what you're willing to fill and what you'd like to receive. Like, last year, I gifted someone a piece of writing, but I received a piece of art from someone. Personally, I always give them three prompts for adult fic, three prompts for genfic, three prompts for adult art, and three prompts for genart, and then the gifter can pick whichever of them they're comfortable with, and I still get to be surprised. I don't believe everyone does this, though, but it's not a bad idea.
I don't know the rules on OCs, though. I'm not sure if they're strictly allowed. Usually it's just canon characters, but rules change from year to year, so who knows, with this one?
And as to taking the reigns on it, I'd love to but I can't. My grandmother's in the hospital this year, and things aren't looking great, so I'd never know if I'd have to drop everything and go to a funeral. If it weren't for that, I'd take it up in an instance, and I still plan to be a participant; just not an organizer.