Deposit bonus is a good idea but the problem is it might be abused since you can easily create multiple accounts I think it can only be applicable to casinos and sports betting websites but probably they could have at least a trading competition on which users can have more wins something, they have existing leaderboard and maybe they can give rewards weekly to top 10 players and restart the leaderboard weekly or have an option to look monthly leaderboard, weekly and competition leaderboard.