There are some things I wanted to say to the OPs and I thought I'd be best to discuss the ground rules.
Here is what's on my mind.
Section A Round Boundaries:
Take a few moments to slice up the last minuet of the round. Establish an end time of the previous round and a few seconds where all deposits are automatically refunded prior to the start of the next.
The losers of the previous round may want to wait till the last second to get at the reward by contributing. List the next address paid out as well as the amount needed to obtain the payout. It may be more than the initial deposit, but if it nets that user a few mBTC they would loose otherwise this could keep a dead round going.
Section B Rolling Addressees:
Don't do this, just stop. The users need to be able to audit your game or else it's like you are actually running a Ponzi.
If you must use a BIP32 Deterministic Wallet that users can verify during the round. Use one wallet per set of game rules.
Security?
Work the wallet backwards so that you can release the current Derived Public Key without releasing any of the next addresses, every round a new DPK is released and it's proven that the next DPK is the one used for the previous round and all rounds that came before.
Typically the first 20 addresses are usable, but you should jump ahead(use a tool like
http://bip32.org/) a few years worth of keys.
