It is not sounding as good idea as it seems you are missing out your big income stream.
The usual practice among all exchanges is, charging for each and every trading and then there will be no withdraw fee (otherwise only network fee).
The market leading Chinese exchanges (Okcoin and Huobi) did this for years. No fees for trading (creating incredible churn and volume), but fees for withdrawals. I have no idea how successful it was from a profitability standpoint, but I believe they only stopped after being pressured by the government earlier this year.