Fencer: Counter proposal:
Set an open ended elimination tournament, with a fixed starting date. On the starting date, all players which are above the 8, 16, 32... limit, in the order of their subscription, are chopped off. So that it's a first come - first play tournament

As MadMonkey rightly points out, trying to guess in advance if you can interest 32 or 64 or 128 people in your tournament is impossible.