Iris gave you some great advice, I've never gone to cons so unfortunately I'm not much help here. Personally I would be looking for some affordable pieces like stickers and other small items you mentioned, and doing sketch commissions on the spot sounds like a great idea.
As for prints, I think it depends a lot on the audience. I know my preferences are somewhat different from your average FiM fan's, I'm rarely interested in show accurate pictures but prefer original styles... Nevertheless, individual portraits or pony pairs like Iris suggested would probably sell, everyone could get one with their favourite pony. Or maybe portrait each pony with their pet? Sorry I don't really have ideas for a large scene, hope someone else will be able to give you some input and ideas!