Ok, I want to input my 5 cents.

I really like the script of weeklyponzi.com/24hourponzi.com and I think it could be a sample for every ponzi. That op really did good job.
According to this I figured out few rules that would make ponzi more comfortable for all
1) One and only one address. People must see all movements of coins.
2) Feedback to users. The main problem of mentioned ponzies is that OP simply didn't give a shit at his players in the end.
3) Instant payout (to prevent double spending at least after 1 confirmation)
4) Strict timestamps according to, for example, blockchain. It means that if person deposited at 23:59:59 - he wont be paid.
5) Ponzi should be time-limited (1-3 days per round. One week period is too slow and 1 day is too risky for someone)
6) Reward for last e.g. 3-5 payments to encourage last users. It can be full payment or just refund.
7) (Optional) Percent growth for each day. For example 1 day - 120% payout, 2 day - 125% payout, 3 day - 130% payout, next round.
8 ) And of course it would be very good to have some reserve fund with ~10 btc

I don't know how honest are you, OP, but I believe you has good intentions about that ponzi and wont scam us. But still you should gain some respect. So don't expect huge activity at start.

Good luck!