I hope the next challenge will avoid too specific a goal. I participated in a couple, but I just wasn't interested enough to devote time to a specific form that was narrowly defined. And at the moment, not interested in tile, though I've made quite a few over the years.
I tend to find the more nebulously defined challenges more thought provoking.
Still, my suggestion is the albarello, or medicine jar. It's a simple form, but can take many shapes and if we're not sticklers for authenticity, many decorative approaches.