I vote for “must have filled out a game concept doc (GCD) and preferably provided some base code to kickstart the project”.
Game concept docs are an essential skill to get used to, and there’s no excuse for not doing them. There’s a guide on Gamasutra to writing 2-3 page ones, but most people for these size games can do all the important bits in less than 1 page - perfect!
Any project that starts with base code - no matter how shoddy - is 20 times more likely to go somewhere. There’s really no other way of effectively weeding out all the projects that will start but then do nothing. It’s sounds harsh, but … how hard is it to write some interfaces and a simple game-loop + some very basic prototype code? Even just a menu screen and no game would only take an evening for a begginer using the resources round here (or under 30 minutes for someone who’s done it all before). If people can’t even stretch that far, whats the chance they’ll do anything on their project?
Just MHO…