This is correct, there can be very many variations. We chose the most basic one, at the same time giving a nod to the original 150% scheme of Mr. Ponzi.
...Since it's trustless the only thing you can compete on is fee, and so it's a race to the bottom. The fee would have to end up at zero.
See also: dice sites, exchanges, and anything else currently taking a fee for a service that can be decentralised and made trust-less.

Hi doog, much respect to you for making just-dice and all the work you do on this forum regarding scammers etc.
I just wanted to chime in and say fee is not the only thing provably fair ponzis can compete on. It can be the mechanisms themselves. For example how often payout? How much. Will there be levels for each investor, can you pay more for a bigger return etc.. There are many things to compete on, the idea is to make it fun and rewarding, and this is just the beginning.