Generally I would not consider joining a project like this, unless you've shown some serious progress. If you really are serious about it, work on it for a couple of months and show off what you got. If you've gotten far enough on your own (which shouldn't be that hard) I think people are more geared to join your cause!
Another idea would be to do a proper design document. I just sat down yesterday and wrote one for a similar game, all in one day (all it should take for a small game really -- everything else is superfluous fluff). The process of deciding exactly what the game is going to be is both rewarding but also highlights obvious flaws in your design.
Anyway, I wish you good luck:)