The best contests are the ones that require creating something original, and not just a picture we post so we can be "picked at random". Contests should be competitive, not based on luck!
I think there should be a contest involving performing for strangers or just out and about, as the primary thing for magicians to do isn't sit in their room and practice moves and cardistry, but to share the magic with laymen.
Something like "best/most original presentation of (enter classic trick or type of trick here)" could be a ton of fun!