Just provide bonus money on weekly basis and the wager requirement like 20 times and 1 week time frame,before credit the winnings to real account, otherwise fortified. Just start on Friday, so that u will have more participants at weekends considering geo time.
Another idea is to predict the btc or eth value against the USD at specific time say 22 CET or GMT on daily basis. Those who are near to the range will get rain .
Hourly btc/USD prediction. Rainbot will rain to the players who predicted close to the hourly btc/USD value or range. If it is predicted exactly, then 25% of rain pool goes the winner, and rest 9 people' will get the balance divided equally. If no winner, 10 players will get equal amount.